Structural and mechanical properties of Al-Si-Mg cast alloys depend on Į-Al phase, eutectic-Si particles, iron-rich intermetallics and especially Mg2Si phase. These can be changed with various ways such as chemical modification, Mg addition. In this study, commercial A360 hypoeutectic Al-Si-Mg alloy with addition of 0.2% - 0.8% Mg, affecting of alteration in microstructure and hardness, was investigated. The alloys in different compositions were produced by gravity casting method. And the casting temperature was set at 650°C. Microstructures were examined and intermetallic compounds were shown in detail. The observations were carried out in MEIJI ML 7100 optical microscopy. According to microstructural results; Mg2Si phase, AlFeMnSi phase and eutectic phase occurred in the structure. In addition, the effect of Mg supplementation in rates of 0% - 0.2% - 0.4% - 0.8% have also examined. The hardness was measured in Brinell macro hardness tester. The results showed that the Mg addition increased the hardness. Micro hardness of intermetallic phases was measured. Thus, the effect of hardness of the phases may be identified. Micro hardness measurements were carried out under load 10 gf in Vickers micro hardness tester. As a result of this work, it was found that Mg2Si phase increased the hardness values. According to the specific hardness values, the hard phase was Mg2Si intermetallic phase which consist of Mg addition. Intermetallics and eutectic Si phase fined with increasing Mg addition. The addition of Mg increased the hardness of the material overall.