The Quaternary basalts in Shandong Province contain mantle xenoliths of varied quantities and sizes. These mantle xenoliths are mainly spinel lherzolite characterized by protogranular and porphyroclastic textures.The dislocation substructures of olivine from the xenoliths indicate a deformation process dominated by dislocation creep under the upper mantle condition. The equilibration temperature and pressure of the xenoliths are estimated to be 800—1100℃and this range of depth, the differential stress of the upper mantle should be 13—26 MPa, while the strain rate and the equivalent viscocity are 10-14—10-20s-1, and 1020—1025Pa·s respectively. The thermal structure and rheological features of the upper mantle beneath the studied area coincide with those beneath the continental extension zone and the rift. It is considered that an upper mantle diapir occurred since Early Tertiary in eastern China, giving rise to the thinning of the lithosphere and the formation of extensional environment in this area. It reached its climax in Late Tertiary and has begun weakening since Quaternary.