In this paper, we presented a rectangular waveguide method to calculate the birefringence of an elliptical inner caldding single mode fiber. The fiber is approximated by a rectangular waveguide, then Marcatili and fective ndex ethod were applied respectively to calculate the birefringence caused by the elliptical clading. Numerical example was given and calculated results were compared with that obtained by plane wave approximation, modified coupled mode theory and perturbation method as well as exact values by solving the eigen value equation. It is shown that rectangular waveguide method is not only accurate but simple.