Microstructure and corrosion behaviour in simulated body fluid of as-cast and heat treated Mg–xZn (x = 3 and 6) alloys for different heat treatment times were studied. The results revealed that as-cast Mg–3Zn alloys consist of Mg12Zn13 phase and α-Mg matrix, while Mg–6Zn is composed of Mg51Zn20, Mg12Zn13 compounds and α-Mg matrix. After heat treatment of Mg–6Zn alloy at 340 °C, the Mg51Zn20 phase decomposed to the matrix and Mg12Zn13 while, the microstructure of Mg–3Zn remained unchanged. The results also indicated that heat treatment at 340 °C has little influence on the corrosion behaviour of Mg–3Zn. In contrast, heat treatment improved the corrosion resistance of the Mg–6Zn alloy as the decomposition of the Mg51Zn20 phase decreased micro-galvanic corrosion. The corrosion resistance of both as-cast Mg–3Zn and Mg–6Zn alloys marginally improved with increasing heat treatment times.