In ultra-precision diamond milling (UPDM), the cutting force as an indicator of machining stability deserves to be discussed extensively. However, most studies have focused on the direct material removal under tool rake cutting, not considering tool flank interference in UPDM. In this study, a theoretical and experimental investigation has been conducted to discuss cutting forces under the tool flank interference in UPDM. Firstly, an analysis model of the interference space between tool flank and workpiece was built up to study the tool flank interference positions. Secondly, a kinematic model for the tool cutting motion was constructed to discuss the instantaneous uncut chip thicknesses (IUCT) under the tool flank interference. Moreover, a cutting force model was proposed to reveal the relationship between the cutting forces and IUCT. Finally, a series of milling tests were carried out in UPDM for the validity of the theoretical results. The theoretical and experimental results revealed that the tool flank interference would take place to deform surface generation and induce sudden changes of the cutting forces in UPDM. The tangential cutting force (Fr) reflects the dominant IUCT, and the radial cutting force (Fr) shows an extremely high sensitivity to the IUCT under tool flank interference, while (Fr) has little correlation with IUCT under tool rake cutting, even negligible. This research work gives a deep insight into the cutting forces with surface generation under the tool flank interference in UPDM.