The effects of Sr on the solidification behaviors of eutectic and hypoeutectic Al-Si alloys under the condition of unidirectional solidification were investigated. The liquid-solid interfaces of the alloys without Sr addition were irregular and the degree of irregularity increased with increased solidification rate. The eutectic silicon of the alloy with Sr addition was not modified at the slow solidification rate of 2㎜/h. The modification effect of Sr was enhanced with increased solidification rate. Meanwhile the unmodified eutectic silicon grew first into liquid and was surrounded by α-aluminum solid solution, the modified one grew in the deviated directions from solidification direction. The Sr was segregated in the eutectic silicon. The undercooling of eutectic Si of hypoeutectic alloy with Sr addition was much greater than that without Sr addition.