Al-Si alloys are the most important aluminium cast alloys because of their superior casting properties such as good castability, low specific gravity, low viscosity and high resistance to wear and corrosion. The effect of strontium (Sr) addition -as a modifier- on the microstructure, hardness and tensile strength properties of the Al-Si alloys at different silicon contents (3.4, 6.7, 9.3, 12.4, and 13.4 %) was investigated. Sr content in these groups ranged between 0.086 to 0.181 %. The nanohardnesss and maximum displacement for Al-Si alloys with different percentages of silicon (6.65, 9.14, 12.23 and 13.5 % respectively) at constant Sr (0.02%) were introduced and strain hardening was detected. The results indicated that the addition of Sr to Al-Si alloy modified the Al-Si morphology from large plates or needle-like silicon structure to fine-scale fibrous silicon structure and hence improved the tensile strength and hardness. The values of hardness and tensile strength of Al- 13-14 Si % were increased from 64.5 to 72 BHN and from 154.8 to 172.4 MPa respectively with increasing the percentage of Sr from 0.086 to 0.17 %. The hardness percentage in nanoindentation measurements was increased by about 26%, while the maximum displacement was decreased from about 7400 nm to about 4930 nm when the Si % increasing from 6.65 to 13.5 %.