With the miniaturization of parts, size effect occurs. The isothermal forming processes are performed to obtain homogenizing deformation. In the paper, a serial of isothermal upsetting tests are carried out with billets of different dimensions. Difference of flow stress is accepted as the parameter to evaluate the size dependence of flow stress on billets dimensions. The experimental results show that size effect occurs clearly. With the increasing of temperature, the difference of flow stress becomes smaller, which means that the degree of size dependence is reduced. Scatter of flow stress is observed in the tests at room temperature. When the deformation temperature is raised, the fluctuation of flow stress tends towards decreasing. In order to investigate the effect of grain size, different grain size is obtained with the heat treatment process. At the same temperature, the difference of flow stress increases with the increasing of grain size. These phenomena can be explained from the viewpoint of polycrystalline structure of material. The anisotropy of individual grain is appeared obviously, which leads the fluctuation of flow stress. In the isothermal deformation, the effect of single grain is smaller than that at room temperature.