Abstract: The phase transformation hardening of cast Zn-Al alloy was studied by Vickers hardness, scanning electron microscope, transmission electron microscope and nano-indentation. The results show that, the cast Zn-Al alloy with aluminum content of 5wt%-25wt% can be hardened when the alloy is treated at above eutectic phase transformation point（277 ℃） for 10 h and cooled at a rate less than 1.2 K·s^-1. The closer the composition of aluminum alloy is to the eutectic phase transition point（Al:22wt%）, and the slower the cooling rate is, the more obvious the hardening effect of the alloy is.With the hardening phenomenon, the alloy structure changes from the granular crystal grains to the layer-like crystal grains.The cast grain contains a steady-state α²phase, and its hardness is about 0.55 GPa. After heat treatment, the lamellar grains are only composed of aluminum-rich α¹phase and zinc-rich η phase, and their hardness is about 0.95 GPa. The decomposition of the metastable phase leads to the phase transformation hardening of the alloy.