Effect of cold rolling on microstructural changes has been investigated for a Zn-15%Al alloy to elucidate the reason for its work softening behavior. Fully annealed microstructure of the Zn-15%Al alloy is characterized by ${eta}$ grains and (${eta}+{alpha}$) lamellar colonies, where ${eta}$ and ${alpha}$ are Zn-rich HCP and Al-rich FCC phases, respectively. The hardness decreases continuously with increasing cold rolling degree, exhibiting work softening behavior. It is revealed that during the cold rolling, (${eta}+{alpha}$) lamellar colonies gradually change into equiaxed ${eta}$ and ${alpha}$ grains due to dynamic recrystallization at room temperature, while pre-existing ${eta}$ grains are only deformed without recrystallization. Furthermore, cold rolling causes the precipitation of dissolved Al solutes in ${eta}$ grains. In view of these results, change of (${eta}+{alpha}$) phases from lamellar to equiaxed morphology, which results in structural softness and increase in equiaxed ${eta}/{alpha}$ grain boundaries with higher mobility, and deterioration of solution hardening by precipitation of Al solutes from ${eta}$ grains, are thought to contribute to the work softening of Zn-15%Al alloy.