In recent years more and more applications are reported about metrology grating, however, almast all the papers were limited to the testing of its accuracy, drift of DC level and phase. As to the testing of its transmission function, nothing more except those theoretical analyses wcre reported. Therefore, a method for calculation of grating transmission distribution by determining its spectral intensity is described here, and the testing data and calculated results about Ronchi-gratings, sine-gratings and their replicated gratings are thcoretically analyzed and calculated. some other problems are also discussed in this paper. With the need for technology and application of gratings, it is necessary to explore further in this field.