Abstract: Through studying the flank face geometry of twist drills, a new concept of effect tail clearance angle is proposed. A method for calculating the effect tail clearance angles is given. The relations between grinding parameters and the effect clearance angles are studied.This paper also shows how to use the new concept as a further criterion to arrive at a solution to the conical grinding method for a drill of given specification. The results show that the tail clearance angle is a very important parameter to drills performance, and the sharpening parameters determine the tail clearance angle. Five sharpening parameters have different effect on the tail clearance angle, θ has the most significant influence and the order of the significance of the other four is C_y、σ、ω、C_x.