The hot deformation behavior and the subsequent microstructure characteristics of Hastelloy G-3 alloy were studied by isothermal compression tests at the strain rates of 0.1-10 s-1 and the temperatures of 1050-1200 oC. The flow-stress at high strain rates were corrected considering the effect of deformation heating, and a new constitutive relationship between peak stress and deformation temperature as well as strain rate for G-3 alloy at high temperature was established. The results show that the built material model has good fitting accuracy to satisfy the engineering need. The softening mechanism of G-3 alloy is dynamic recrystalization during hot working process. According to the microstructures after deformation, for G-3 alloy the proper deformation temperature is 1180-1200 oC and the proper strain rate is 5-10 s-1.