The structure aberrations of color separating grating (CSG), which is caused by the fabrication process, generate great modulations to the laser beam when CSG is used in the final optical system of a high power laser system. The beam modulations may lead to severe optical damages to CSG itself and the system. In this paper, a comprehensive fabrication error model for CSG is developed to study the beam modulation characteristics in the near field. Through theoretical calculations and simulations, the near field diffraction pattern of CSG is obtained, and the relationship between beam modulations and CSG structural aberrations is presented. The results show that beam modulations are mainly caused by the stair depth error and the slope error. This study provides a convenient way to estimate the possibility of optical damages induced by CSG. (c) 2005 Published by Elsevier B.V.