Abstract: The effects of weathering on organic matter of black shales has been studied using organic geochemistry, scanning electron microscopy and X-ray fluorescence energy spectrum. The black shales were collected from the Lower Cambrian Yuertusi Formation along Dong'ergou section in Kalpin area, Tarim Basin. As weathering increased, organic abundance became much smaller, and hydrocarbon generation potential became poorer. The maximum weathering losses of organic matter and chloroform bitumen "A" were more than 95% in black shales and less than 50% in siliceous rocks, indicating that siliceous rocks were more resistant to weathering. Climate and surface exposure time were the main causes for rock weathering. Mineral composition and organic matter occurrence determined weathering degree. Free organic matter occurring in over-mature shales was easily lost through weathering. In petroleum evaluation, it is important to not only study the present geochemical features of source rocks, but also to considerweathering effects.