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The rheocast structure in a hypoeutectic high chromium white iron was studied. By analyzing the microstructure of the special subgrain boundaries, it was found that the solute enriched special subgrain boundaries, usually thought to be the boundaries of agglomerated separated grains, were in fact evolved from bent thin roots of dendrite arms. Further examination of the relationship between cooling rate and grain size revealed that a dendrite arm with a thin root could be bent or sheared off. At the cooling rate that favors the formation of thin rooted long dendrite arms, a fine rheocast structure can be gained. (C) 2002 Elsevier Science B.V. All rights reserved.