This paper presents a novel method for evaluating the resolution of a plane diffraction grating. The phase of wavef ront diffracted oy a grating under test is measured with interferometry and a graph of 3D)-shape of the diffracted wavef ront is obtained. The the Fourier transform is carried out by the algorithm of FFT and the intensity distribution in spectral diffraction pattern and the half width of the spectral image are determined. The advantages, fundamental principle and the experimental technique of this novel method are described, the results of experiments and computation are presented too. They are in good agreement with the resul obtained by the method commonly used.