Abstract: On Nov.6,1988,two strong earthquakes with M7.6,M7.2 occurred respectively in Lanchang and Gengma county,Yunnan Province.The tectonic rapture distributed in the limestone region along the two sides of Xiaohejiang River in the epicentral area.The stress tensors reversed from the rupture zone with fault slip vector method are:the azimuth of σ₁ is 223°,the dip angle is 28°;the azimuth of σ₂ is 42°,the dip angle is 62°;the azimuth of σ₃ is 132°,the dip angle is 0°;the ratio of stress R=0.57.As a whole,it shows slip and compressive character,which is consistent with the stress field character indicated by the focal mechanism.The analysis to the fault slip vector can be as a method to determine whether a ground fissure is tectonic.