This paper deals with exit burrs produced in drilling. The development of yielded, deformed and strain-hardened area is investigated respectively by Finite Element Method(FEM), Grid Method(GM) and Hardness Distribution Measurement(HDM) to clarify the burr formation mechanism. In FEM, the drill exit part of workpiece is divided into 198 triangle elements and 119 nodes. For GM and HDM, aluminum alloy(A6030 in JIS) plates of 10mm thickness are drilled under various cutting conditions with drills of 8mm diameter and various helix angles. And also the measurement of cutting forces is done at the same time. The plastic deformation of workpiece starts at the drill point part, and then occurs mainly at the outer corner part. Both plastic deformation and burr become small as the helix angle increases. Burr shapes are divided mainly into two types. One is a small even burr with or without a conical hat chip. The other is a large burr shaped like a petal. The difference of the two burrs is caused by the formation of a conical hat chip.