Abstract: Effect of the added reinforcement on the nucleation processing of aluminum alloys was studied by a differential scanning calorimetry (DSC). The onset temperature measurement of monolithic alloys and composites reveals that nucleation of the aluminum phase is restricted with the presence of the reinforcement. In addition, the undercooling of α-Al phase increases with the decreasing of inter-fiber spacing. So α-Al phase cannot preferentially nucleate and grow on the reinforcement surface. In contrast to α-Al phase, the undercooling of primary silicon decreases with the presence of the reinforcement, which shows that the reinforcement can act as preferential nucleation sites for the primary silicon. It is effective to study the nucleation phenomenon of aluminum metal matrix composites by the DSC.