Four kinds of star couplers with different structures at rib-to-slab and slab-to-rib transition region on silicon-on-insulator are fabricated. The testing results are consistent with the theoretical calculation in which transition loss about 1 dB by using taper waveguides was obtained. The loss is caused by mode dismatching between the rib waveguides and slab waveguides. Polarization effect in SOI rib waveguides is also discussed. Experimental data in the released literature are quoted to validate the analysis method. And the effective index difference which is as low as 10-5 order between two polarizations of single mode rib waveguides is obtained by optimal design for the rib waveguide thickness, rib height, rib width. In fact this so little effect can be neglected in practical design.