Small amount of calcium addition to the Mg-9A1-0.8Zn-0.2Mn (AZ91) alloy resulted in the obvious influence on mechanical properties. The yield strength of the alloys increased with the increase of Ca addition and maximum strength was obtained from the alloy containing 0.15% of Ca. The creep resistance at the temperatures between 150-200℃ was also significantly increased with Ca addition. The creep rate (at 200℃, 50MPa) of the alloy with00.15% of Ca addition was one order of magnitude lower than that of the base alloy (AZ91). Microstructural observations revealed that the addition of calcium refined the microstructure and enhanced the thermal stability of the β precipitates, which accounted for the improvement of creep resistance at high temperatures. Small amount of calcium addition to the Mg-9A1-0.8Zn-0.2Mn (AZ91) alloy resulted in the obvious influence on mechanical properties. The yield strength of the alloys increased with the increase of Ca addition and maximum strength was obtained from the alloy The creep resistance at the temperatures between 150-200 ° C was also significantly increased with Ca addition. The creep rate (at 200 ° C, 50 MPa) of the alloy with 00.15% of Ca addition was one order of magnitude lower than that of the base alloy (AZ91). Microstructural observations revealed that the addition of calcium refined the microstructure and enhanced the thermal stability of the β precipitates, which accounted for the improvement of creep resistance at high temperatures.