The morphology, microstructure and decomposition behavior of M2 C carbides in high speed steels with different chemical compositions have been investigated by scanning electron microscopy, transmission electron microscopy, electron backscatter diffraction and X-ray diffraction.The results show that the morphology and substructure of M2 C carbides are very sensitive to chemical compositions of high speed steels.M2 C carbides present the plate-like shape in tungsten-molybdenum steel and present the polycrystal orientation in the eutectic cell.In contrast, they show the fibrous shape in molybdenum-base steel and exhibit the monocrystal orientation.Plate-like and fibrous M2 C carbides are both metastable and decompose into M6 C together with MC at high temperatures.MC nucleates inside the plate-like M2 C while it is formed at the fibrous M2 C/matrix interface during the decomposition process.Such differences are expected to arise from different compositions of plate-like and fibrous M2 C carbides.