Abstract: An analytical expression of diffraction efficiency of color separation grating (CSG) was deduced from the scalar diffraction theory and Fourier optics. Moreover, the relation between diffraction efficiency and phase delay was analyzed. If the phasedelays brought by different parts of CSG meet corresponding conditions, the incident wave will be diffracted into one of the three central diffraction orders with highest efficiency. In addition, some errors, including etched depth error and duty cycle error, induced by two different fabricating processes were analyzed and compared. In ICF studies, CSG, which are fabricated by the two processes, have the same fluctuation range in diffraction efficiency for the third harmonic when tolerated etched errors are the same. Duty cycle errors can bring