To analyze rib waveguides with trapezoidal cross se ction, a simple numberical analysis is presented which uses effective index meth od and transfer matrix theory. Results of the propagation constants and the coup ling coefficients between modes in trapezoidal-cross-sectioned rib waveguides show that they have the same action with wider rectangular-cross-sectioned rib waveguides. This will simplify the designing of trapezoidal-cross-sectioned r ib waveguides and increase the precision of practical device.