Analysis of instantaneous uncut chip thickness (IUCT) in peripheral milling of curved surface with variable curvature is nontrivial due to the combined influences of both process geometry and cutter runout. This paper gives a systematic analysis of IUCT including the effects of changing workpiece geometry and the cutter runout in peripheral milling. The prominent feature of this analysis procedure lies in that the novel equation for computing the IUCT is mathematically derived in detail. Numerical simulations are performed to study the effect of workpiece curvature and cutter runout on IUCT. The proposed model is validated by comparing the measured cutting forces with those predicted based on the IUCT which is obtained using the current approach.