In some situations, such as machining sculptured surfaces with long or slender ball-end mills, the cutter deflection in ball-end milling is a main factor affecting the machining accuracy. In this paper, a mechanistic cutting force prediction model of ball-end milling in consideration of cutter deflection is presented and its machining characteristics are discussed with experimental results. This model takes account of the changes in chip thickness by the deflection of a cutter and the cutter deflection is obtained by solving an equilibrium equation between cutting force and reaction force. The model was verified from the measurements of cutting forces for different machining conditions.