Based on the semi-vector wave equation under cylindrical coordinate system,the mode distribution and complex propagation constant in bent waveguides were computed by a finite difference method with perfectly matched layer(PML) boundary condition.The radiation loss was obtained from the imaginary part of the complex propagation constant and the transition loss was calculated by two-dimension overlap integral method.In comparison with the experimental results by Deri,the calculation result is rather accurate.The relationship between bending loss and the structural parameters of SOI rib waveguide was analyzed by the method presented,and the offset at the junction of bent and straight rib waveguides was optimized as well.