The wetting behaviors of R(01 (1) over bar2) single crystalline and polycrystalline alpha-Al2O3 substrates by Al-Si alloys were studied over wide composition and temperature ranges. The wettability is quite good for all compositions of the alloys. The effect of temperature is moderate while that of the composition is significant. The dependence of the wettability on the alloy composition displays a "valley" profile with the minimum value appearing in the range of 60-70at.%Si. The wetting improvement by the addition of Al to Si mainly results from the decrease in the solid-liquid interfacial free energy by the Al segregation at the interface, while that by the addition of Si to Al results from the decrease in the surface tension of the liquid by the Si segregation.