The effects of Sr addition on the microstructure and mechanical properties of in-situ ZrB2 nanoparticles reinforced AlSi9Cu3 composites synthesized via direct melt reaction were systematically investigated. The addition of Sr could modify the morphology of eutectic silicon particles and refine the second dendrite arm spacing of α-Al dendrites. Altatively, the ZrB2 nanoparticles were also refined and distributed more homogenously, and nanoparticle ag-glomerations were reduced with the addition of Sr element. The results demonstrated that the addition of 0.06 wt％of Sr element can reach the optimal refinement of the second dendrite arm spacing of α-Al and the modification of eutectic Si. In this case, the ultimate tensile strength, yield strength and elongation reached 330 MPa, 226 MPa and 15.70％, respectively, which were remarkably enhancement in comparison with the base alloy.