Abstract: Passive seismic tomographic imaging is a new and economical exploration method as compared to the traditional seismic reflection method because passive seismic tomographic imaging only needs observations directly from micro-earthquakes occurring underneath the study area, while the reflection method needs firing shots at the surface. So this work is an attempt to use the passive seismic tomographic imaging technique to study deep tectonics beneath an area of -100 km^2 in an oilfield in the western Qaidam basin. The preliminary results based on a total of 3289 P-wave arrival times of 626 seismic events show good coincidence with the known large tectonics of the study area. A very striking feature imaged by the proposed model is a NW-trending large anticline. In addition, the micro-earthquakes are also in the main located along the active fault belt.