The shear stress in an unsaturated soil increases rapidly with limited shear strain to a peak value and then drops gradually with a further increase in the shear strain until a residual value is reached. In other words, there is a significant strain-softening behavior under large shear deformation. A variety of geotechnical structures (e.g., slopes, foundations, retaining walls and piles) associated with unsaturated soils typically undergo a large progressive deformation prior to reaching failure conditions due to the influence of environmental factors (e.g., rainfall infiltration and wetting-drying cycles). As a result, the shear strength of soils in sliding zones typically reduces from a peak to a residual value with the progressive development of large shear deformation, while the shear strength of soils in other zones are still at the peak level. In other words, in many scenarios the strain-softening behavior of unsaturated soils can significantly influence the mechanical behavior of geo-structures. Therefore, a thorough understanding of the shear strength behavior of unsaturated soils during strain-softening is required to reliably interpret the mechanical behavior of geo-structures that undergo large shear deformation. Significant advances have been made during the last thirty years to understand and model the strain-softening behavior of unsaturated soils. Most of these studies however focus on the strain-softening behavior within a relatively small shear deformation due to the limitations of the experimental apparatuses. Only limited experimental studies under large shear deformation were reported based on the modified suction-controlled ring shear apparatus. Therefore, more investigations are still required to provide a comprehensive understanding of the shear strength behavior of unsaturated soils during strain-softening under large shear deformation. Studies presented in this thesis are directed towards investigating the shear strength behavior of unsaturated soils during strain-softening and its application in geotechnical engineering practice. The following studies have been conducted: (i) A state-of-the-art review of the strain-softening behavior of unsaturated soils published in the literature during the past three decades is summarized. The physical mechanisms and modelling methods of the strain-softening behavior and the peak, critical and residual shear strength of unsaturated soils are investigated. (ii) A disturbed state concept model is proposed to predict the variation of shear stress in unsaturated soils during strain-softening process under drained condition. Five sets of experimental data gathered from the literature on unsaturated soils varying from coarse- to fine-grained soils are used to verify the proposed model. The proposed model can provide reasonable predictions for the strain-softening stress-strain relationships of various types of unsaturated soils. The model is simple in concept and all the required parameters can be obtained from conventional saturated and unsaturated shearing tests and pressure plate tests. (iii) Two sets of suction-controlled multistage ring shear tests are conducted on unsaturated SP-SM soil and Indian Head till (IHT), respectively. The variation of the shear stress, void ratio, and water content of specimens during shearing (the shear displacement reaches 100 mm) under multi levels of net normal stress and matric suction are described and discussed. The influence of matric suction and net normal stress on the residual shear strength envelops of unsaturated soils are critically discussed. (iv) A model for predicting the residual shear strength for a wide range of unsaturated soils comprising coarse- to fine-grained soils is developed in terms of two stress state variables (i.e., the net normal stress and matric suction) by using the soil water characteristic curve as a tool. The model is formulated and validated based on experimental data in a series of suction-controlled ring shear tests using the axis-translation technique, including the two sets of tests (SP-SM and IHT) conducted in this research and another three sets of tests (SM, SC-SM and CH) gathered from the literature. The fitting parameters are related to the plasticity index (Iₚ); thus, only four basic parameters (i.e., cᵣ', φᵣ', Sᵣ and Iₚ) are included in this approach. (v) A series of slope stability analyses of a landslide in unsaturated condition are conducted using Geoslope software based on the peak and residual shear strength parameters. The analyses results highlight the role of residual shear strength in the slope stability of unsaturated soils. In summary, the mechanical behavior of unsaturated soils under large shear deformation is comprehensively investigated in this thesis. The experimental results of the suction-controlled ring shear tests reported in this research contribute towards understanding the fundamental shear strength behavior of unsaturated soils during strain-softening under large shear deformation. The models proposed in this research provide simple tools to predict the shear strength of unsaturated soils under different levels of shear deformation.