Being products of water-rock interaction during the migration of fluids, clay minerals can help us to understand the mechanism of water-rock interaction and features of fluids. With the Jinshan gold deposit in Jiangxi Province as an example, the authors determined the crystallinity, mean crystal size and lattice strain of illite and chlorite and, on such a basis, discussed the difference of crystallinity, mean crystal size and lattice strain between illite and chlorite caused by deformation of mylonite and untramylonite. There exist strong negative correlation between crystal size and crystallinity of illite in altered mylonite of low strain and low water/rocks ratios, and positive correlation between crystal size and crystallinity of chlorite in altered untramylonite of high strain and high water/rocks ratios. From altered mylonite to altered untramylonite, the crystallinity of illite decreases whereas that of chlorite shows no obvious variation with the increasing strain. It is held that the formation of clay minerals is affected not only by water-rock interaction but also by the strain rate and recovery rate during rock deformation. The relationship between crystallinity and crystal size of illite or chlorite can reflect the behavior and mechanism of deformed rock.