With the miniaturization of parts, size effects occur in the micro-forming processes. To investigate the effects of the specimen size on the flow stress, a series of upsetting deformation experiments were carried out at room temperature for specimens with different diameters. And the grain size of billets was changed by anneal processes to analyze the grain size effects on the size dependence of flow stress. The deviation of stress was observed. The results show that the flow stress decreases with decreasing billet dimensions. As the dislocation accumulation in free surface layer is slight, the reduction degree of flow stress becomes larger when the plastic deformation goes on. The flow stress is enlarged by grain size, which can be analyzed by the grain boundary length per area. The deviation increases with decreasing specimen size. This can be explained by the effects of grain orientation stochastic distribution according to the Schmid law. As a result, the micro-forming process must be considered from the viewpoint of polycrystalline structure, and the single grains of micro-billet dominate the deformation.