Microstructure and mechanical properties of Mg-6.0wt%Zn-0.5wt%Zr (ZK60) alloy were studied as a function of cooling rate. The temperature field and cooling rate during the casting process were investigated by use of finite element analysis (FEA) simulation. The results showed that the microstructure was refined and the eutectic phase distributed much uniformly with the increase of cooling rate. The increase of yield strength, ultimate strength and elongation can be ascribed to the strengthening effect of fine grain. Relationship between grain size and yield strength is consistent with the Hall-Petch formalism: 1/ 2 80.37 132.56 − = + d y σ .