Abstract: A rigorous coupled-wave (RCW) method, which adopts scattering matrix to calculate the diffraction efficiency of blazed gratings, is proposed to point against the low-accuracy of general scalar approximate algorithm. The calculation of diffraction accuracy was fulfilled by establishing a thorough theoretical model of blazed gratings and solving each harmonic coefficient with scattering matrix. The method has been used for calculating the diffraction accuracy of both TE and TM modes and solving the problem caused by numerical instability. The scalar method based on Beckmann-equation is provided for comparison.