A study for investigating the geometric characteristics of the side wall, which is generated by the flat end-milling process, is carried out through experiments and geometrical analysis. For this research, flat surfaces of prismatic parts are considered. It is assumed that the change of material removal per tooth causes the change of tool deflection. Under this assumption, it is verified that the milled surface geometry is directly affected by the amount of material removal per tooth. Analytical models for predicting the location, size, and depth of the geometrically defected zone are also developed.