Abstract:The Zhangqiang area has undergone 4 tectonic phases since the Late Jurassic: the early Middle, late Middle and Late phases of the Yanshanian, and the late phase of the Himalayan. By using the AE method, the authors have measured the corresponding maximum principal stress values of the 4 phases, which are 106. 2 MPa, 87. 3 MPa, 57. 1 MPa and 33. 4 MPa, respectively. Meanwhile, it has been determined that the present -day maximum principal stresses range between 17. 4 to 23. 3 MPa. It is also found that the values of palaeostress are larger for the tectonic phases accompanied by magmatic activities, while those of the present-day stress are smaller for oil -bearing structures and oil producing wells in the oil fields. The maximum principal stress occurring in the formation phase of petroleum accumulation in the area is 87. 3 MPa.