The crystalline structures of zinc oxide (ZnO) formed by the internal oxidation of a Pd-Zn alloy were examined at elevated temperatures. Metastable sphalerite ZnO with a tetrahedral shape preferentially nucleated in the Pd matrix, while plate-like precipitates consisting of a wurtzite ZnO phase preferentially grew at a high temperature. Unique ZnO precipitates with trapezoidal cross-section and consisting of inter-layered sequences with sphalerite ZnO and wurtzite ZnO were also examined at an intermediate temperature. It is inferred that the formation of stacking faults in the sphalerite ZnO is strongly related to the nucleation of the wurtzite-type ZnO sequence.