In diamond fly-cutting of microstructured surface, the cutting force plays a crucial role in the establishment of cutting plans, the setting of cutting conditions and the determination of the characteristics of cutting processes like tool wear and surface texture. The characteristics of cutting force are introduced and a mechanistic cutting force model for predicting the cutting force in diamond fly-cutting of microstructured surface is proposed in this study. In the model the cutting edge radius of single crystal diamond tool is considered. Furthermore the contribution of elastic deformation of the workpiese to the total force is also accounted. The force components in the horizontal 'X' and 'Y' direction are calculated from material hardness, elastic modulus, uncut chip area, tool geometry and the average value of shear angle respectively. And the predicted cutting forces are consistent with experimental cutting force.