The optimization for fabricating large cross section Si/GeSi/Si single mode rib waveguides with RF heated CVD method is discussed. The optimal fraction x of Ge in GeSi layer defined by the necessity of single mode, numerical aperture and critical thickness is between 1～3%. The height and width of rib must be also limited by the single mode and large cross section, The etched rib side walls are slope due to the Si crystal structure. The best initial width of rib is 5～6 μm. The etching and polishing solutions must ensure the rib side walls and end faces to be mirror. The measured results show that the performance of the optical waveguides improred remarkably. The output light field is increased and the propagation losses is less than 0.5 dB/cm.