A computer program for the determination of the degree of crystallinity and the disorder parameter in lactose based on Ruland's method, one of the X-ray diffraction methods, was written. The correctness of the program was confirmed by comparing the computed results with those caloulated by the usual Ruland's method and Herman's method using several samples of lactose with different crystallinities. A series of integral upper limits was determined by rearranging the equation presented by Ruland. The degree of crystallinity and the disorder parameter in crystalline lactose which was used as the 100% crystalline standard in the other methods were estimated as ca. 70% and ca. 3.5 A², respectively. The effects of the normalization constant on the calculated results were rather small. However, the effects of integral upper limits on those results were large. A series of integral upper limits seems to be a very important factor for crystallinity determination by Ruland's method.