Continuous casting has been widely applied in the production of steels and other metals. However, it has been rarely used in producing high speed steels, which are still manufactured by the conventional method of mould-casting. Thus, little is known about the microstructure of high speed steels made by the continuous casting technology. In the present work, AISI M2 steel is produced by horizontal continuous casting and the difference of solidification microstructure of ingots by different casting technologies has been examined. The results show that the networks of M2C eutectic carbides are greatly refined in the ingot by continuous casting compared to that by mould casting. Meanwhile, the morphology of M2C eutectic carbides changes from the plate-like type to the fibrous one, due to the increasing cooling rates. Compared with the plate-like M2C, the fibrous M2C in continuous casting ingots is less stable and decomposes faster at high temperatures, spheroidizing obviously after heating and refining dimensions of carbides.