Fault gouges are direct products of faulting and hence record a great deal of information on fault activity. In this paper, the chemical composition, crystal pattern and structure of the illite from fault gouges at Xiaowan, Yunnan, are studied by XRD, microscopic observation and chemical analysis. The studies are synthesized as follows. (1) An oriented alignment of illite generally develops in fault gouges, kink band or fold frequently develops in the aggregates of illite, forming a flow structure in between clastic grains. (2) The interlayer ions (2Ca+Na+K)≈1 within the illite form fault gouges is higher than that within the illite of weathering and marine origin, the crystal water (H+O) within the illite from fault gouges is lower than that within the illite weathering and marine origin and MgO within the illite from fault gouges is higher than that within the illite of weathering and is lower than that within the illite of marine origin. In conclusion, the illites from the fault gouges at Xiaowan are different from the illites of weathering and marine origin. The illite from fault gouges is products of the lower grade metamorphism due to the fault motion. (3) XRD analyses show that illite from fault gouges at Xiaowan belnog to ISII structure: △[(002)-(001)] >8°;Ir >1; BB1 <4°; 1 M pleomorphism; b0 value of illite is 8.993Å; The patterns interlayer positive ions[2Ca+Na+K]≈1 ;which show that the growth of illite from fault gouges are of low pressure and temperature (<200℃). (4) The frictional heating caused by rapidly faulting may lead to chemical and physical changes occurring in some minerals from the fault and even silicate minerals are fused. The order ISII structure and 1 M pleomorphism of the illite from fault gouges show that the growth of illite are of low temperature (<200℃), and characters of low temperature and microstructure of the illite from fault gouges show that the fault motion at Xiaowan is creep slip.