The flow stress behaviors and the microstructure characteristics of 690 alloy with different grain sizes were investigated by isothermal compression tests in the temperature range of 1100~1200 °C and strain rate range of 0.1~10 s-1 on thermo-simulation machine. The flow stress constitutive relationship with initial grain size parameter of 690 alloy was established. The results show that increasing of initial grain size produces higher values of flow stress and the critical strain for the onset of dynamic recrystallization. Dynamic recrystallization volume decreases with increasing of grain size. The flow stress of 690 alloy predicted by the proposed models well agrees with experimental results. Therefore, the constitutive equation of 690 alloy during hot deformation process is improved