Burrs farmed in drilling are classified into three types, no burr, burrs with cap, teared burr. To control burr size in drilling, the second type burrs with cap are to be formed because it is small and uniform. It is necessary to understand the mechanism of cap formation to derive the burr formation into second type burr with cap. In several materials. second type burrs are formed in drilling by changing cutting conditions. It is observed that cap is formed as a result of the plastic deformation along the outside of exit hope. According to the tension behavior of the material in concentrated region between hole and drill outside edge, the geometry of burr with cap is determined. Simplified 2D FEM analysis shows good prediction for burr formation.