A parametric study is performed to investigate the parallel tunnel interaction. Three dimensional finite element analyses were performed to determine the effects of soil stiffness, pillar width and advancement level of the second tunnel on the behaviour of displacement, bending moment and shear force of the previously constructed tunnel. In the analysis PLAXIS 3D Tunnel geotechnical finite element package was used. This program allows the user to define the actual construction stages of a NATM tunnel construction. In the analysis, construction stages are defined in such a way that firstly one of the tunnels is constructed and the construction of the second tunnel starts after the construction of the first tunnel. The mid-length section of the first tunnel is investigated in six different locations and at seven different advancement levels in terms of displacement, bending moment and shear forces. It is found that, displacement and bending moment behaviour are more related with soil stiffness and pillar width than the behaviour of shear forces. While the level of advancement of the second tunnel causes different type of responses on the shear force behaviour, level of advancement does not affect the type of behaviour of displacements and bending moments. Another finding of the research is that pillar width has an evident influence on the behaviour of displacements and bending moment than the soil stiffness. It is also found that the interaction effect may be eliminated by increasing the pillar width equal or larger than an approximate value of 2.5 &ndash / 3.0 D (diameter) for an average soil stiffness value.