Comparsions of single mode waveguide conditions for single mode silicon on insulator (SOI) and GeSi/Si rib waveguides obtained by the effective index method (EIM) and proposed by Soref et al. with the experimental result are carried out. It is found that single mode waveguide condition obtained by effective index method is in good agreement with Rickman′s experimental result. Conditions for multimode rib waveguides loading the first and the second order mode are also obtained by effective index method.