The creep behavior of the as-extruded Mg-8Gd-1Er-0.5Zr alloy was studied in the present paper under various temperatures (150-200 ℃) and stresses (50-70 MPa) for 100 h. The microstructure evolution during creep was investigated by optical microscopy (OM) and transmission electron microscopy (TEM), and the creep mechanism was analyzed. The results show that the alloy exhibited good creep properties under the experimental conditions. The creep curves can divide into two stages: the deceleration creep stage and the steady creep stage. The steady-state creep rate is 6.48×10_-11 s_-1, and the creep strain is 0.007% at the temperature of 150℃ and the stress of 50 MPa; The steady-state creep rate is 4.26×10_-9 s_-1, and the creep strain is 0.226% at the temperature of 200 ℃ and the stress of 50 MPa. In the case of lower temperature (150 ℃), diffusion mechanism acts as the main control mechanism, whereas dislocation mechanism controls the creep behavior at the higher temperature(175 ℃, 200 ℃). Furthermore, the precipitates of β′ phase in grains and the β phase at grain boundaries are formed during the creep process. The orientation relationship between the β′ phase and α-Mg matrix is (020)_β′∥_Mg, [001] _β′∥[0001]_Mg. The β′ phase can effectively inhibit the dislocation gliding, and the β phase can pin gain boundaries, both of them play an important role in improving the high temperature creep performance of the alloy.