This paper presents an effective tool path generation method for machining of free-form surfaces of molds and dies with ball end mills. Parametric surfaces are generally adopted to represent free-form surfaces of products by progress of CAD systems. The proposed method is to generate the tool path immediately by referring to the parametric surface equation such as NURBS form. Therefore the tool path is generated easily even when the geometry of the cutting tool changes. It can also generate accurate cutter locations without any offset approximations. And furthermore the tool feed rate can be controlled properly at the cutting point on the product surface. The fundamental procedure of the proposed tool path generation method is described here and some cutting simulations are executed for the verification of the proposed method.