Abstract In micro cutting, both the cutting edge radius and the material grain size have a great influence on the cutting force. The study of the influence of the material grain size on the cutting force is unavoidably affected by the dual effect of the cutting edge radius. In this paper, a method for estimating the shearing force is proposed. In addition, micro turning experiments were performed to investigate the influence of the cutting edge radius and the material grain size on the cutting force. The results showed that a smaller material grain size leads to a larger cutting force and a higher specific cutting energy. Furthermore, the difference in cutting force observed for different material grain sizes increased with the cutting edge radius. By separating the shearing force, this dual effect was found to be caused by the ploughing force. The influence of the material grain size on the cutting force and the specific cutting energy is much lower than the influence of the cutting edge radius.