A new cutting force model of micro-milling was developed based on calculating the instantaneously projected area of cutting in this study. According to the rotation trajectory of cutting edge, the instantaneous cutting area projected to xy-plane was first divided into several portions and determined with rectangular integral. After the cutting coefficients were experimentally investigated, the cutting force of micro-milling with different cutting conditions can be determined. Because the developed cutting force was function of cutting parameters, such as spindle speed, feed rate, depth of cut, with taking the cutter deflection into account it was further used to the determine of optimal cutting parameters for obtaining better cutting efficiency and machining accuracy. To verify the proposed cutting force model and the way to determine the optimal cutting parameters, micro-milling experiments were conducted. The results have shown the feasibility of the proposed model and method.