There has been a significant research interest in metal micro-forming for the last ten years. However, neither a methodology nor an exact micro mechanical property has been well determined. Hence, several experiments are carried out in this research to point out the influence of “size effect”. Based on the comparison between the experimental data and relevant parameters, the material characteristics in micro scale can be found. In order to find the mechanical properties in micro scale, this study shows the effect of specimen size and grain size on the micro tensile test of copper material. Firstly, five copper micro-sheets with different thicknesses have been chosen as experimental materials. Since the minimum standard test piece formulated in ASTM is still too big for this micro tensile test, this study decided to shrink the sample size of specimen shape to 1/2 . Then the flow stress decreases with decreasing specimen size. This way is able to get the important factor which influence the flow stress on the micro tensile test. Secondly, these samples of different grain sizes are made by heat treatment. Then these samples of different grain sizes are used to conduct the experiment. According to the experimental result, the change of flow stress, which is influenced by various grain sizes, can be found. Finally, the change in the mechanical behavior between specimen size and grain size is obtained.