The purpose of this study is to determine the most efficient working condition of superfinish for cast iron, and to study the cutting mechanism of the superfinish. As a result of a systematic research on the various working conditions, such as abrasive grain, grain size, stone pressure, frequency and amplitude of the stone, surface speed of the work and viscosity of the cutting oil for the two-stage superfinish of cast iron, the following conclusions were obtained. (1) As the abrasive grain, GC grain is more suitable than WA grain. (2) For the first stage of superfinish, surface speed of the work of 13 m/min, stone pressure of 1.5∼2.0kg/cm2 and viscosity of the cutting oil of 1.7 Engler degree are suitable. For the second stage of superfinish, work speed of 34m/min, stone pressure of 1.0∼1.4kg/cm2 and the viscosity of 2.3 Engler degree are suitable. Concerning the grain size, the frequency and the amplitude of the stone, the same values are applicable for both cases. (3) The two-stage superfinish is effectively applicable for ground surface whose roughness is higher than 2μ. (4) By microscopic observation, it was found that the chip of superfinish takes the form of shear type. With the use of the above working conditions, it is possible to get a mirror-finished surface which is smoother than 0.1μ for cast iron.