Microspheres (< 1000 μm) have applications in various fields (e.g., drug delivery, cosmetics, food, etc.). Microspheres can be generated by the micro-fluidic technique, in which microspheres are produced from one fluid under the action of another immiscible fluid in a network of channels. There are four performance indexes associated with a microsphere generation process with a device, namely (1) the size of microspheres (as small as possible), (2) the uniformity of size distributions (as high as possible), and (3) the flexibility of devices (i.e., the size range of microspheres that can be generated with one device), and (4) the efficacy of the microspheres generation process (mass production or not). Two operating principles along with their corresponding devices, the modified T-junction device and membrane emulsification device, are studied in this dissertation, because of their unique features, with the former having an excellent task flexibility and the latter having an excellent efficacy. The study defined three objectives, namely (1) understanding the mechanism of the microsphere generation process with the modified T-junction by both numerical investigation and experimental investigation, (2) optimizing the microsphere generation process with any micro-fluidic device in general and the modified T-junction device in particular (optimization: the size and uniformity), and (3) designing and fabricating a new emulsification membrane by tackling the shortcoming (i.e., fragile with the membrane) with the existing emulsification membrane. For objective (1), a simulation model was built first, validated by the experiment, and then the simulation model was employed to study the regimes. For objective (2), a new optimization procedure was first proposed for general micro-fluidic systems and then applied to the modified T-junction system. For objective (3), a new membrane was designed and fabricated and tested. The following conclusions can be drawn from the study: (1) the modified T-junction device works based on a combined operating principle (flow focusing and conventional T-junction) and there are three regimes (instead of the four regimes in the conventional T-junction) in the flow; (2) the optimization of the microsphere generation process makes sense for the micro-fluidic device in general and the modified T-junction in particular (the optimal modified T-junction is: the mean size: 16.1 μm and 24.8 μm, and the uniformity (Standard Deviation (SD)): 0.2 μm and 0.7 μm); (3) the shortcoming with emulsification membrane can be overcome with a multi-layer membrane architecture. There are several contributions made by this dissertation in the field of micro-fluidic. First is the provision of an accurate Computational Fluid Dynamics (CFD) model for the modified T-junction. Second is the new knowledge discovered regarding the mechanism of microsphere generation with the modified T-junction device. Third is the provision of an effective optimization approach for any micro-fluidic device in general and for the modified T-junction device in particular. Fourth is the design with the successful fabrication of the membrane emulsification device based on new system architecture (i.e., multi-layer structure). From an application’s perspective, this dissertation has provided evidence that with the micro-fluidic technique, the smallest size of microspheres can be 2.3 μm; the highest uniformity (SD) can be 0.8 μm. Further, if an application puts emphasis on the task flexibility, the modified T-junction device is an excellent choice, and if an application puts emphasis on the mass production, the multi-layer membrane device is an excellent choice.