Abstract: The characteristics of tectonic stress tensors are studied with over 700 focal mechanism solutions in the three Datong earthquake series from 1989 to 2002.The results show that the tectonic stress in the vicinity of hypocenters was relatively strong before the three main shocks.Though there were some slight differences in the directions of stress after the three events,the maximum principal compressive stress directions are basically consistent with the direction of tectonic stress field in North China,except for the one before the earthquake occurred in1999.The direction of maximum principal compressive stress before 1999's earthquake was 226°(in SW direction),which may be the result from the joint action of local tectonic stress field and that of in North China.