X-ray diffraction measurements have been carried out for two kinds of samples of CVD-amorphous silicon nitride with different values of the density. From radial distribution analysis, it became clear that the short range structure in these samples resembles that of the crystalline β-phase. Coloring in CVD-amorphous Si 3N 4 was found to be induced by X-ray irradiation. Based on the deficient numbers of the first nearest neighbours in SiN bondings, it was proposed that CVD-amorphous Si 3N 4 consists of small clusters including vacancies in their matrix.