The influence of cooling rate and Fe-containing phases on Sr-modification of Si phases in hypoeutectic Al-Si aloys, a problem with great industrial importance, was investigated. The microstructures of samples were examined using scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDX). A new method of electron probe microanalysis (EPMA) map scanning was used to analyze the Sr distribution, which gave quantitative results covering more Si particles. EPMA map scanning, together with SEM with EDX, was also used in analyzing the distribution of Fe phases. Results show that Fe-containing phase was related to the unmodiifed Si particles in samples with partial modiifcation failure and the plate-like Si phases in samples without modiifcation failure. Such a relationship was further conifrmed by the microstructure observation. In conclusion, a partial failure of Sr-modiifcation can be caused by both slow cooling rate and Fe-containing phases.