The Mg-12Gd-2Y-Sm-0.5Zr (wt. %) alloy was prepared by casting technology and the microstructure and mechanical properties of the alloy have been investigated. It was found that the alloy was composed of a-Mg matrix, Mg5Gd phase and dispersed long-period ordered β' precipitates. With temperature increasing from 20 to 300 °C, the tensile strength increased and reached to the maximum strength at 300°C, namely 316.3 MPa. The value was superior to those of the conventional heat-resistant magnesium. The improved strain hardening and critical crack propagation stress were the key factors for experimental alloy possessed anomalous temperature dependence of tensile strength.