Abstract: The constant of grating is one of the most important parameters to assess the quality of grating.A new method to measure the constant of grating is discussed in the paper. The information of a diffractive image is captured by planar-array CCD.The constant of grating can be got by calculating the power ratio of the main facula to the ±1~astafacula.With the advantages of the quick detection, simple structure and high precision,the method can be used in real-time measurement of the quality of gratings in production process.