In as-cast structure of AISI M2 high speed steel, M2C carbide prevails, the morphology of which has crucial influence on distribution and dimension of carbides in final products. In this study, the morphology and microstructure of M2C formed at different cooling rates have been investigated by scanning electron microscope, X-ray diffraction, transmission electron microscope, and electron back-scatter diffraction. The results show that the morphology of M2C carbide changes from the plate-like type to the fibrous one with increasing cooling rates. Surprisingly, the microstructure between plate-like and fibrous M2C is significantly different. Twining and stacking faults are observed in the plate-like M2C, which is supported by great misorientations between adjacent carbides. However, no planar faults are identified in fibrous M2C and the carbides in one colony have almost identical orientation. It is expected that plate-like M2C grows as a faceted phase, while fibrous M2C formed at high cooling rates is likely a non-faceted phase. The difference of liquid/solid interface structure is supposed to result in the morphology change of M2C.