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.