The present work has investigated the effect of aluminum on the solidification microstructure of M2 high speed steel. The results show that the as-cast structure is refined and the networks of M2C eutectic carbides are distributed more homogeneously with the addition of aluminum. Meanwhile, the morphology of M2C changes from the fibrous shape to the plate-like one. Despite of the same crystal structure, the microstructure differs greatly between the two carbides. Compared with the fibrous M2C, micro-twining is formed in the plate-like M2C, resulting in the orientation changes between different plates. It is expected that the solid/liquid interface structure of M2C may evolve from the non-faceted into the faceted due to the addition of aluminum.