Abstract: The microstructure during solidification of high-carbon steel billets was investigated, The results showed that primary dendritic arms in the billets were becoming coarser and shorter with evolvement of solidification. A higher carbon content in the steel was favorable to form short and thick primary dendrites as well as wider primary dendritic arm spacings (PDAS) and coarsening columnar structure. A higher superheat promoted coarse primary dendrites and wider PDAS. With the withdrawal rate slower, it was easy to form narrow PDAS. The greater the secondary cooling water flow ratio, the easier the spindly primary dendrites and narrow PDAS formed. Mole-electromagnetic stirring (M-EMS) greatly reduced the PDAS and dendritic length-to-width ratio. With the increase of M-EMS electric current, the PDAS was reduced obviously.