In this study, the effects of Zn additions on the mechanical properties of Al-Si-Mg-Cu alloys were investigated by increasing the amount of Zn up to 8wt.%. As the Zn content was increased up to 6 wt.%, the yield strength and elongation changed linearly without any significant changes in the size and shape of the main reinforcement phase. However, it was confirmed by SEM observation that the Mg-Zn phase formed between the reinforcement phases when the amount of Zn added exceeded 7wt.%. A Mg-Zn intermetallic compound formed between the Mg2Si phase, becoming a crack initiation point under stress. Thus, the formation of the Mg-Zn phase may cause a sharp decrease in the elongation when Zn at levels exceeding 7 wt.%. It was also found that the matrix became more brittle with increasing the Zn content. From these results, it can be concluded that the formation of the Mg-Zn intermetallic compound and the brittle characteristics of the matrix are the main causes of the remarkable changes in the mechanical properties of this alloy system.