The effects of silicon additions up to 3.5 wt% on the as-cast carbides, as-quenched carbides and as-tempered carbides of high-speed steels W3Mo2Cr4V, W6Mo5Cr4V2 and W9Mo3Cr4V have been investigated. In order to further understand these effects, a Fe-16Mo-0.9C alloy was also studied. The results show that a critical content of silicon exists for the effects of silicon on the types and amount of eutectic carbides in the high-speed steels, which is about 3wt%, 2wt% and 1wt% for W3Mo2Cr4V, W6Mo5Cr4V2 and W9Mo3Cr4V, respectively. When the silicon content exceeds the critical value, the M2C eutectie carbide almost disappears in the tested high-speed steels. Silicon additions were fotmd to raise the precipitate temperature of primary MC carbide in the melt of high-speed steels that contained δ-ferrite, and hence increased the size of primary MC carbide. The precipitate temperature of primary MC carbide in the high-speed steels without δ-ferrite, bowever, was almost not affected by the addition of silicon. It is found that silicon additions increase the amount of undissolved M6C carbide very obviously. The higher the tungsten content in the high speed steels, the more apparent is the effect of silicon additions on the undissolved M6C carbides. The amount of MC and M2C temper precipitates is decreased in the W6Mo5Cr4V and WgMo3Cr4V steels by the addition of silicon, but in the W3Mo2Cr4V steel it rises up to about 2.3wt%.