The single mode condition for a rib waveguide based on silicon-on-insulator (SOI) is determined through the eigen mode analysis using a finite difference method. This single mode condition gives the relation between the normalized rib height r and the normalized rib width t. A comparison is made between the present result obtained with a FDM and previous analytical formulas for the single mode condition. Simulation results show that the value for the constant in the previous analytical formulas should be chosen as 0.3 when r >0.5. The present single mode condition is also extended to the range of r<0.5 (which is not considered before).