The microstructe of mold fluxes of low-carbon steel and mould powders were researched by means of polarizing microscope. The result shows that the mineral compositions of the mould fluxes primarily are glass phase, wollastonite, quartz, melilite, pyroxene and feldspar. The main mineral compositions of mould powders is melilite. The quartz, pyroxene and glass phase are helpful to reduce the crystalline portion of mould powder, but the melilite, wollastonite help to improve the crystalline portion and promote crystallization of melilite. It is good for the crystalline portion when the casting speed droop.