The process of pure copper micro-cylinder upsetting is performed by a micro universal testing machine. Through the heat treatment and precision wire cutting technology, the specimens are designed with different dimensions and grain sizes. The effects of grain size and specimen size on the flow stress are investigated, respectively. It is found that the flow stress decreases with the miniaturization of the specimen size and the relationship between flow stress and volume fraction of surface grain is linear. The effect of grain size on the flow stress is indistinct as the specimen is larger, but the flow stress of the specimen with larger grain size is obviously lower than that with small size. The surface of the micro-forged specimens show significant inhomogeneous deformation as the specimen size is smaller. The size effects due to the changing of specimen size and grain size can be explained by surface model and fine grain strengthening theories. Meanwhile, the scale parameter is introduced to quantify the size effect and thus the linearly changing of follow stress with the volume fraction ψ can be explained.