Abstract: The Yaoan，Yunnan, MS6.0 earthquake occurred on July 9，2009. By inverting seismic phase arrival time data and using the double-difference tomography approach (Zhang and Thurber), the aftershocks were relocated and a 3-D P-wave velocity structure around the earthquake source region were obtained. After data selection 873 aftershocks were used for inversion. The relocation result shows that aftershocks distribute in a belt with WNW-ESE trend. Focal depth of most aftershocks is between3 kmand12 km. By analyzing the change of focal depth with time we infer that the stress in the focal region underwent three time adjustment. The stress was adjusted from bottom to top in the first 40 hours，and then was adjusted in a larger depth range in the following 20 hours. After 60 hours, the stress adjustment stopped below the depth of12 kmand the stress was released mainly in the shallow crust. The obtained 3-D velocity structure indicates that the mainshock occurred in a transition zone between low and high velocity bodies. The aftershocks mainly distribute in the velocity transition zone and by the side of the high velocity body. Combining with the relocation result，we infer that there exists a buried fault with ~N65deg;W strike in the transition zone between low and high velocity anomalies.