The evolution of microstructures during the solidification of 20SiMn steel was investigated. The dominant role of coa-lescence was revealed by analyzing the quenched microstructures and the solute distribution. In the parameter range investigated, including the slow solidification, the fast solidification and the directional solidification, the coalescence can happen at multiple solidification stages and on multi-scales. During the directional solidification, the tary and fourth dendritic arms coalesce into the secondary dendritic arms, and the secondary dendritic arms further coalesce to produce the cellular crystals. The cellular crystals become coarse through further coalescence. During the non-directional solidification, the initial microstructures are discrete crystals without the dendritic patt. Both the dendritic arms and dendritic stems appearing in the later solidification stage are produced by the coalescence on multi-scales. The coalescence is more significant in the later solidification stage. The mechanism of coalescence is attributed to different melt compositions in different local domains and the reduction of the interface energy.