The change of microscopic structure of a polished surface of steel during gas carburization was observed. The results obtained were as follows: (1) By gas carburization, grain boundaries on the polished surface of steel appeared even in 30 seconds at the temperature as low as 650° and new grains began to grow. (2) The carbon content of the surface reached the near eutectoid composition quickly, and it increased gradurally up to the saturated concentration of austenite.
