By means of optical microscope (OM), field emission scanning electron microscope (FE-SEM) and series properties tests, the effects of the mixed-grain micromicrostructure on mechanical property of nickel - based superalloy GH4720Li was studied in this paper. The results were as follows, the tensile strength and yield strength at 650℃ for specimens with portional local coarse grain are slightly lower than that with uniform fine grain under the tensile tests at high temperature. The tensile strength drops rapidly with the increasing of volume percentage of coarse grain. Furthmore, the deformation mechanism of superalloys at high temperature is not the same under different stress-rupture test conditions.Under the temperature and stress condition of 730℃/530MPa, the stress-rupture life increases with the increasing of volume percentage of coarse grain for the reason that grain boundary sliding mechanism plays an important role. While the major deformation mechanisms is found to be dislocation gliding under 680℃/830MPa. Therefore, the stress-rupture life decreases with the increasing of volume percentage of coarse grain. Under this condition, the specimens with local coarse grain and fine grain have the best stress-rupture property. Grain boundary sliding also have certain effect under the condition, which results in not obviously reduction of the stress-rupture life for specimens mainly with coarse grain.