3D compression ramp flows are computed to investigate side wall effects on the shock wave / boundary layer interaction. A Reynolds-Averaged Navier-Stokes code and a boundary layer generation code are applied for the generation of inflow boundary conditions. The skin friction and the pressure coefficients on the centerline of the 3D ramp are compared with those from 2D computations. The computation results are validated with the experimental data and a three-dimensionality caused from the side-wall effect is investigated.