Abstract: Distributions of the focal depths in the Peking area and such provinces as Shanxi,Shandong,Lingxia and Yunnan for 1976 Tangshan,1975 Haicheng,1970 Tonghai,1966 Xingtai and 1976 Longling earthquake sequence were studied using more accurate earthquake data.The results show that the foci scatter within a range of depths from 4 km to 30 km which is called an active layer of the earthquake and are concentrated mainly in a layer of thickness of 10 km—15 km which is called a dense layer of the earthquake.For various region,different are the depths at which the seismically active dense layer remains A comparison between distributions of foci and crustal structures was made,showing that the foci are mainly located in upper part of the crust.The characteristics of focal distributions are determined by the depth at which a transition of brittle shearing fracture to ductile deformation occur.The differnce of the temperature and pressure as well as the type of rocks may account for that of depth foci between the dense layers for differsnt areas.It appears that some foci can be found to be contoured,represented by some lines on a section of crustal structures among which is C discontinuity,as a result that a part of a discontinuity serves as a surface along which stick slip may appear.The earthquake tend to attend those areas where the thickness of the crust changes sharply for the reason that there have been a lot of preexisting faults and concentrations of stresses in the gradient belt in quenstion,which would have made a favourable condition for occurring of earthquakes.