This paper proposes an efficient method to the lowest cost tool sequence for semi-finishing of mold cavity on a 3-axis mill machine. Suppose that the rough machining has completed, so the redundant slice is thin enough and can be removed with a single pass of the semi-finishing cutter. To guarantee the cutting operations do not gouge the desired surface, there is a largest cutter which can be used at any part of the surface. On the other hand, to improve efficiency, large cutter should be priority selected. Dynamic programming finds a near optimal tool sequence based on the approximate evaluation of machining cost.