The effect of aluminum on the solidification microstructure of M2 high speed steel, particularly the morphology and microstructure of eutectic carbides, has been investigated by OM, TEM, SEM, EBSD and XRD. The results show that the as-cast microstructure consists of dislocation martensite and M₂C eutectic ledeburite. Excessive amount of aluminum, 1.2%, favors the formation of ferrite and needle-like carbides. After the addition of aluminum, eutectic carbides are distributed more homogeneously. Additionally, the morphology of M₂C eutectic carbides transforms from the fibrous to the plate-like, and their microstructure also changes significantly. The plate-like M₂C has crystal defects, such as micro-twins and stacking faults, and different growing orientation between adjacent plates whereas the fibrous carbides have few defects and single crystal orientation. Compared to fibrous carbides, the plate-like carbides are much difficult to get spheroidized at high temperature, which is unfavorable for carbide refinement. The ferrite, formed by adding excessive amount of aluminum, cannot be eliminated by ordinary heat treatments, decreasing remarkably the hardness of high speed steel after quenching.