Magnesium alloys have emerged as potentially good candidates for numerous applications, especially in automotive industry. Although the commonly used magnesium alloys, such as AZ91 and AM60 based on Mg–Al system have excellent castability, good room temperature mechanical properties and low cost, the application of these alloys has been limited to temperatures below 120℃ because of their poor heat resistance, especially creep property at elevated temperatures. Recent development reported that magnesium alloys with high zinc and low aluminum concentrations (Mg–Zn–Al based alloys) exhibit better creep properties than Mg–Al based alloys at temperatures above 150 ℃ and small amounts of alkaline–earth element (Sr and Ca) additions to the Mg–Zn–Al ternary alloys lead to further improvement of their mechanical properties. The purpose of the present paper is to describe the effects of calcium and strontium additions on the microstructure and mechanical properties of Mg–12Zn–4Al–0.3Mn based alloy. The results indicate that the as–cast microstructure of Mg–12Zn–4Al–0.3Mn alloy consists of the α–Mg matrix and a quasicrystalline Q phase at grain boundaries. Small amounts of Sr addition to the master alloy result in the transition of metastable Q phase to the equilibrium Mg₃₂(Al, Zn)₄₉ phase and the formation of binary eutectic phase Mg₅₁Zn₂₀. Another lamellar eutectic phase Al₂Mg₅Zn₂ is observed in the as–cast microstructure when Ca combined with Sr is added to the base alloy, and its volume fraction increases with increase of Ca addition. The single addition of Sr causes the increase of tensile strength, but decrease of creep resistance at elevated temperatures. However, the creep properties are significantly improved if Sr is added in combination with Ca to the master alloy due to the formation of ternary eutectic phase Al₂Mg₅Zn₂, which shows the high thermal stability at elevated temperatures. The alloy with composition of Mg–12Zn–4Al–0.2Sr–0.4Ca–0.3Mn exibits good creep resistance at 175℃and 70 MPa.