Abstract: The two contacting surfaces of basalt, granite, diorite and andesite sinter at 350℃-900℃ tempera-ture and 1×10⁸-4×10⁸Pa confining pressure after 1-35 days and have a very high shear strength but smaller than that of original rock.This shear strength increases with the raise of temperature, confining pressure and time.In the condition that the angle between sintered surface and principal compressive stress is larger than 55°, the sintered surface does not fracture but shear fracture occure along the other orientation.In the course of sintering, the new mineral is not producing and the mineral melting effect is not occurring either, but the density of rocks increases and the crystalline grains grow up.The adhering effect of crystalline grains and the volumetric diffusion of atoms are the physical mechanism of sinter-ing.The spontaneous change of surface energy of crystalline grains from high to low is the basic motive power to make for sintering.Ruptrue of sintered surfaces are the principal mechanism of recurence of earthquakes in fractrue zone in the deep earth crust.The experimental lowest temperature and confining pressure correspond to 10 km depth in earth crust.This is the dividing line between the fracture in the upper part and the sintered layer in the lower part of the deep large fracture.After loading, the deep large fracture extends down and forms earthquake source in the lower part.This is likely the principal cause of that the deep major earthquake sources occured at 15-25km depth in the earth crust.