The microstructure and crystallography of M7C3 carbide in 20 wt.% Cr hypereutectic cast iron have been investigated. The results show that primary M7C3 carbide subjected to fast cooling displays an irregular hollow hexagonal structure. Rapid solidification has remarkable effects on the morphology and orientation of primary M7C3 carbide. Transmission electron microscopy (TEM) results indicate that M7C3 carbide has hexagonal structure and crystallizes on {011 0} planes with some stacking faults generating along [0001] orientation. The twins in M7C3 are formed on (011 0) twin plane accompanying selfperpetuating sub-steps which can impellingly promote M7C3 carbide growth. The chemical formulas of primary and eutectic M7C3 carbides are determined to be Cr3.78Fe3.22C2.99 and Cr3.75Fe3.25C2.99, respectively. High-resolution transmission electron microscopy results reveal that primary M7C3 carbide emerges outcrop and promptly grows after nucleating in liquid phase. The existences of sub-steps and stacking faults show that crystalline defects can act as an important role in M7C3 carbide growth. Moreover, an outflanking growth model of primary M7C3 carbide is discussed based on the crystalline defects and growth condition, which results in the formation of hexagonal carbides. (C) 2015 Elsevier B.V. All rights reserved.