Order structure of illite-montmorillonite mixed-layer mineral was judged by coefficient of variation,based on mathematical statistics theory.Through the X-ray diffraction,characteristic peak height of illite-montmorillonite mixed-layer mineral was obtained.Being statistical object,these data were progressed in statistical calculation by coefficient of variation method.It was shown that coefficient of variation of 6 samples was below 0.75.Mixed-layer structure of illite-montmorillonite in all samples was regular mixed-layer.Difference coefficient method is rapid and accurate for the judgement of orderliness of illite-montmorillonite mixed-layer mineral,compared with interval coefficient method.