Mg-10Gd-0.5Zr alloys with 1-5wt% Sm addition were prepared by melting and casting method. The effect of Sm on microstructures and mechanical properties were investigated by tensile tests, SEM and XRD. The results show that the Mg₄₁Sm₅ phase is found because of Sm addition, Sm promotes the precipitation of Mg₅Gd. After solution-aging treatment, the coarsening eutectic structures at grain boundary were dissolved and the precipitation dispersed homogeneous. The finer grain size is observed when content of Sm come to 3wt%. Some of Mg₅Gd phase become short rod-like when Sm addition is 5wt%. Under the same experimental temperature, the strength of tested alloys increases at fist and then decrease, and peak strength present when content of Sm is 3wt%. For the same alloy, with increasing of test temperature, the strength increase at fist and then decrease too. When Sm content is 0~1wt%, the temperature of tensile strength peak is 200℃ and the temperature is 250℃ when Sm content is 3~5wt%. There are abnormal temperature effects in tensile tests, the effect is reinforced and the temperature of tensile strength peak rise with the Sm addition.