In order to investigate the effect of Ni on the high-temperature strength of Al-Si cast alloys, tensile properties of hypoeutectic and eutectic alloys were determined at 250 °C after long-term annealing at test temperature. LOM- and SEM-analysis revealed the existence of Al3Ni-phases in close contact to eutectic Si. It was shown that the strength can be increased by the addition of Ni, though just to a certain level, depending on the fraction of eutectic phase in the alloy. The alloys were considered as a coarse two-phase system where a hardening effect is caused by load transfer to the harder phase, which requires a certain connectivity/contiguity of the latter. The paper describes the extent of contiguity of the eutectic as well as the hard silicon and Al3Ni-phases within the eutectic, and discusses their contribution to an enhanced strength of Al-Si alloys at elevated temperatures.