Abstract: Taking advantage of seismic data obtained from wide angle reflection and refraction profiles passing through this area,the velocity structures and tectonics of the crust and upper mantle in Datong-Yanggao earthquake province and its surrou nding area have been studied in details.The results show:the crust and upper mantle velocity structures and tectonics obviously present both longitudinal and la teral heterogeneity.A great number of basement faults lie in the shallow crust to which the deep structures correspond.According to the characteristics of waves groups,the interfaces of interior crust and the undulations of the velocity contour as well as the boundaries of the low velocity abnormal blocks,deep crustal faults are inferred.The most obvious low velocity block in upper crust lies in the neighbourhood of Datong-Yangyuan and in its south boundary exists deep crustal fault.Datong-Yanggao earthquake swarm is related with this low velocity abnoumal block as well as the deep faults.