In this study, the size dependence of metallic materials on the plastic deformation behavior was investigated in microcoining process. In order to conduct experimental works, a miniaturized microforming system was first developed. Then, a series of microcoining experiments were performed in a wide range of process conditions. The effects of grain size and feature size were analyzed by relating the protruded feature height to the size factor ${lambda}$. It was found that the ratio of channel width to grain size is the critical factor to identify the forming performance in microcoining process. As the size factor decreased below 10, the feature size effect became prominent due to the fact that the reduced number of grains in the deformation region causes the reduced degrees of freedom in slip systems. By observing the protruded feature of the formed parts, the deformation characteristics and underlying physics associated with the miniaturization effects can be discussed.