The purpose of this paper is to acquire the knowledge of a skilled machinist in respect to polishing operations of injection mold cavities, in order to develop a high performance polishing machine. Polishing experiments with different kinds of polishing tools are carried out not only by a skilled machinist's handwork, but also by mechanical method. The polishing properties of a skilled machinist in respect to surface roughness, stock removal and scratch are obtained and analyzed quantitatively. The surface roughness decreases exponentially with the increase of polishing times. The surface roughness (Ra) depends on the following threeactors : roughness before polishing (Ro), final surface roughness (Re) and exponential coefficient (a). The stock removal curve can be represented approximately by two lines and the polishing times corresponding to the intersection of the two lines coincide with the critical polishing times in which the surface becomes better by a degree of 90%. Differences of surface roughnesses between hand polishing and mechanical polishing are small being only 10%. However, the rate of stock removal to polishing times in hand polishing is large being about twice that in mechanical polishing, and the rate of scratch to roughness in hand polishing is kept constant with 4 for different kinds of tools and smaller than that in mechanical polishing.