The effect of silicon on the eutectic carbides in as-heated M2 high speed steels is investigated by using optical microscope, scanning electron microscope, energy dispersive spectrometer and X-ray diffraction analysis. The high speed steel ingots with different silicon additions are produced by electroslag remelting process (the mass content of silicon is 0.3%, 0.8%, 1.6% and 2.4%, respectively). The ingots are annealed at 780 ℃ for 10 h and heated at 1 165 ℃ for 2.5 h. The results show that the eutectic carbides distributed in the dendrites with continuous or semi-continuous net-shape in the as-annealed ingots. The eutectic carbides change from lamellar-like M2C to herringbone-like M6C with the increase of silicon content. After 2.5 h heating at 1 165 ℃, the lamellar-like M2C decomposes into granular M6C and MC, and the size of granules in M2 high speed steel ingot with 0.8%Si addition is smaller than that in the 0.3%Si. Whereas herringbone-like M6C is unchanged during heating treatment, and coarse primary blocky carbides which are harmful for properties still exists after the deformation.