A new method,Profile-fitting of X-ray diffraction spectroscopy (PFXDS) method,which was adopted to determine the size distribution of nanocrystalline grains,was introduced in this paper.Its theoretical model and basic hypothesis were clearly interpreted and its advantages and disadvantages were discussed in details.The PFXDS method was applied to the size distribution calculation of nano-SnO2 powder produced by sol-gel technology and the relatively ideal results were obtained.