Due to the cylindrical type of the end mill, tool deflection is one of the main problems in surface generation. Using FEM Analysis, relations between tool shape parameters and rigidity are revealed. Tool stiffness of the end mill is represented as equivalent diameter and determined by several geometrical factors that describe shape of the end mill. Because the relation between equivalent diameter and each factor is not analytic, neural network is used. For good result, the structure for back propagation algorithm is determined by trade-off. The simulation result using neural network is verified by comparing with experimental result.