Abstract: An earthquake with magnitude 4.8 occurred on December 22, 1974 in the Shen Wo area, Liaoning province. From the analysis of various data concerned, it was known as a typical reservoir induced earthquake.No earthquake occurred at the Tanghe reservoir, only over 20km far away from the Shen Wo reservoir, after the impounding in 1968. Through the comparision of the various conditions in the two reservoirs,discussions are directed to the cause of the Shen Wo reservoir induced earthquake. Our opinions are that the well-developed faults, the fracture of rock- mass and the good conditions for the water penetration are the most important factors causing the reservoir induced earthquake.