The mineral composition and microstructure of slag film of medium carbon steel are studied systematically with the methord of the technological mineralogy. The result shows that: The main mineral compositions of slag film are melilite and glass phase, and the texture of slag film is of two-layer or three, in which glassy-layer and crystallizing-layer appear alternately, the crystallization rate is from 2% to 5%. The melilite in slag film is good for improving lubrication and controlling heat transfer, so melilite can be promote in slag film for improving the quality of medium carbon steel.