Effects of Zn and Zr additions on the microstructure and mechanical properties of Al-5%Mg-1%Mn alloys were investigated. As Zn content increased in the Al-Mg-Mn-Zn alloys, the tensile strength and ductility of as-cast alloys rather decreased while the tensile strength of the heat-treated alloys significantly increased mainly due to the precipitation of fine $MgZn_2$ phases. Small amount of Zr was added to the 3%Zn alloy to further enhance the mechanical properties, and it appeared to increase the strength and ductility, especially in as-cast state.