The steady state photocarrier grating (SSPG) technique has been used for determining diffusion length of carriers in amorphous hydrogenated silicon (α-Si:H). The novelty of the presented paper is consistent in the taking into account in SSPG investigations not only the spatial distribution of radiation internally reflected in a sample but also the influence of parameters describing continuos distribution of electron states (DOS) in the mobility gap of an amorphous semiconducor. Therefore, the SSPG measurements allowed to determine energy levels and densities of accepto- and donor-like states in the investigated α-Si:H.