Long crack fatigue tests were carried out on two cast Al-Si alloys used in the manufacture of pistons for small automotive engines. These were performed at a frequency of 15Hz at room temperature, 200 degreesC and 350 degreesC, in addition further tests were performed at 50Hz at the highest temperature. The fatigue performance of both alloys was found to improve with increasing temperature. This was linked to a change in fatigue crack propagation mechanism in both alloys with respect to the primary Si phase. It was seen that the transition between a given Si phase decohering or fracturing ahead of a fatigue crack occurred at a temperature and strain rate dependent critical stress intensity factor. The relation between critical stress intensity factor, temperature and strain rate differentiated the two materials studied.