Based upon equivalent transmission line method, a perturbation analysis on the guiding properties of refractive-index gratings is presented. By numerical calculation, the leakage parameter α which influences the leakage energy into the diffracted orders due to the grating scatter, is achieved as functions of the grating period, thickness of the waveguide and index modulation Δ n. Our results reveal that, although the index modulation Δ n is much less than that of the etched or deposited grating, refractive-index gratings can still reach a high leakage parameter because of the prominent increase of the grating height.