The effect of Ni on the thermal conductivity and fluidity of a low Si-added Al-Si casting alloy was investigated. The room temperature thermal conductivity of an Al-2 wt%Si alloy instantly dropped when adding 0.5 wt%Ni, whereas further additions of Ni up to 3 wt% had little influence on the thermal conductivity, which was in the range of 180-185 W/mK. The thermal conductivity was also estimated for the alloys cast at various cooling rates by measuring the electrical conductivity using the Wiedemann-Franz law and increased nearly linearly with an increase in the cooling rates. At such a low level of 2 wt%Si, the castability of the Al-Si alloys, which was evaluated by a fluidity spiral test, was enhanced by the Ni additions, exhibiting the maximum fluidity length at 0.5 wt%Ni.