Abstract: This study presented a method for estimating stress magnitude in earthquake source region based on rotation of stress axes, seismic stress drop and relative magnitude of principle stresses. We applied this method to the Homestead Valley fault segment of the 1992 Landers earthquake, and obtained the compressive, intermediate and extensional principal stress before the mainshock being 271, 266 MPa and 259 MPa, respectively. The preseismic normal and shear stress are 265 MPa and 6.2 MPa, respectively. The post-seismic normal and shear stress are 266 MPa and 1.6 MPa, respectively. The result shows that normal stress increase after the mainshock would have benefited to the rupture termination. The pre-seismic shear stress is higher than that of post-seismic one, and their difference corresponds to the stress drop approximately. The estimated underground compressive stress is very large, but the differential stress is small, and the shear-stress projected on to the fault plane is also relatively small.