The microstructure of permanent mold cast zinc alloy ZA27 was examined by SEM and TEM after natural aging for 18 months. It was found that the primary a’ phase, peritectic and eutectic phases all decomposed into the equilibrium well-formed a+r| lamellae or irregular a+T| structure through cellular reaction. The cell colonies nucleated on the interdendritic eutectic T| layer and grew into the primary dendrites, thus first making the phase in the dendrite edges decompose, and then the a’ phase in the dendrite cores. The products from the a’ phase appeared in regular lamellae rather than irregular particles. In addition, a fine, dense transitional phase a’m containing 27.8 mass% Al, with a fee crystal structure and lattice parameter of about 0.4013nm, formed in lamellae. Copper was preferentially concentrated in Zn-rich n phase and existed in the form of Cu-rich e phase (CuZo,) particles, with hep crystal structure and parameters a=0.274nm, c=0.429nm and c/a=l .567. It was found that the primary a ’phase, peritectic and eutectic phases all decomposed into the equilibrium well-formed a + r | lamellae or irregular a + T | structure through cellular reaction. The cell colonies nucleated on the interdendritic eutectic T | layer and grew into the primary dendrites, thus first making the phase in the dendrite edges decompose, and the a ’phase in the dendrite cores. In addition, a fine, dense transitional phase a’m containing 27.8 mass% Al, with a fee crystal structure and lattice parameter of about 0.4013 nm, formed in lamellae . Copper was preferentially concentrated in Zn-rich n phase and existed in the form of Cu-rich e phase (CuZo,) particles, with hep crystal structure and parameters a = 0.274 nm, c = 0.429 nm and c / a = 567.