Hypereutectic Al-Si alloys have many advantages such as excellent wear resistance, high ratio of strength to density, good castability (esp. fluidity) and elevated strength. However, in casting process, the segregation of coarse primary Si and flake eutectic Si decrease mechanical properties. In this study, using AlCuP refiner and Sr modifier, it is intended to obtain simultaneous effects of refinement of primary Si and modification of eutectic Si in hypereutectic Al-Si alloys, to measure hydrogen gas contents and to evaluate mechanical properties. Experimental results show that in the case of simultaneous effect of refinement of primary Si and modification of eutectic Si, two step addition that modifier is added into melt after refiner addition at 750℃ is more effective than simultaneous addition of modifier and refiner at 750℃ in terms of microstructure and hydrogen gas control. All mechanical properties tested were found to be improved by refinement of primary Si and modification of eutectic Si.