Abstract： This paper discusses the electromagnetic field distribution in phase grating and the rate of diffraction for different diffraction orders by using a rigorous modal theory. Piece wise analytical function is used to express the eigen functions instead of the infinite series in other methods. Stable, fast and accurate recurrent formulae are developed to calculate the reflection transmission and amplitude coefficient matrix that can treat phase grating with any number of groove steps. The numerical results show that there exist bands of total reflectance. The conditions and the physical mechanisms for which the total reflectance occurs are discussed.