On 14 November, 2001, a great earthquake of Ms 8.1 occurred on the Kunlun fault in the Kusai Lake segment of the Kunlun Mountain, Qinghai Province in northwest China. The stresses measured at the sites near the Kunlun fault before and after the earthquake show a great drop after the earthquake. The values of the horizontal maximum principal stress at two sites are 12.9 Mpa and 6.8 Mpa before the earthquake. Measurement of the horizontal maximum principal stress at the same sites after the earthquake, showed values of 3.5 Mpa and 2.2 Mpa respectively. This is only about one-third of that before the earthquake. Apparently stress that accumulated near the fault before the earthquake was released along with the rupturing of the surface. The direction of the maximum horizontal principal stress also varies before and after the earthquake. The direction is found to deviate from that of the regional structure stress field after the earthquake. Such stress changes offer a basis for predicting large earthquakes in the region.