Abstract: Diffractive optical elements (DOEs) have been widely applied to acquire different beam shapes in illumination systems. However, the experimental values of diffraction efficiency are greatly different from the ideal values because of stray light of high diffractive orders around the targeted spot. The reasons were theoretically discussed for the stray light and many DOEs with different design parameters were simulated. Through the simulation analysis, a new parameter, named relative period, was defined. It was proportional to the wavelength and inversely proportional to the far-field diffraction angle and the size of DOE cells. It is suggested that the diffraction efficiency, which is independent from the beam shapes, could be improved by increasing relative period. As a result, the stray light of high diffractive orders could be suppressed effectively by regulating the parameters of the far-field diffraction angle and the size of DOE cells. The new parameter has an important role in the design of DOE.