Materials have different characteristics of absorption or transmission of light according to the wavelength of light. Materials can be analyzed with spectro-photometer which is used to measure the absorbance of materials. The Stray light is all of the unwanted light reaching the detector. In spectrophotometer, stray light is the light except the light of the analytic wavelength and the source of measurement error of absorbance. Some experimental results showed that diffractive grating is the major factor of stray light in spectrophotometer. Through the ray tracing with the software tool, classified the paths of the diffractive light from grating and found the range of wavelength which reach the exit slit. The quantity of the stray light(0.025%) is more than the minimum limit of stray light(0.01%) of the single monochromator. A novel optical layout design method, which prevents the reflected rays entering the diffractive grating and minimizes reincident ray upon the diffractive grating, is proposed.