Abstract: The effect of Nb on the as-cast microstructures and the prespheroidization at elevated temperature of eutectic carbides of M2 high speed steel was studied. The experiments show that the block carbides increase with Nb content rise at fast cooling rate,and that when the Nb content is small the Nb carbides with particular shape would be precipitated during solidification at slow cooling rate,however, the Nb is not found in the matrix or the lamellar M₂C carbides. The results also indicate that Nb has no effect on the prespheroidization of M₂C plate carbides, but the prespheroidization of Nb carbide is tool diffficult.