The effects of bismuth and antimony additions on the microstructure and mechanical properties of AZ91 alloy have been studied. Results show that a small amount of bismuth or antimony additions to AZ91 increases the yield strength and creep resistance significantly at elevated temperatures up to 200°C. The highest creep resistance has been obtained from the alloy with combined additions of bismuth and antimony. The activation energies of the steady-state creep for AZ91-based alloys studied were close to that of pure magnesium self-diffusion, indicating that dislocation climb is responsible for the creep mechanism under the present conditions. Microstructural observations reveal that the additions of bismuth or antimony have the effect of refining the β (Mg 17 Al 12 ) precipitates in as-cast alloys and suppressing discontinuous precipitation of the β phase effectively during the aging process. Some rod-shaped Mg 3 Bi 2 or Mg 3 Sb 2 particles distributed mainly at grain boundaries have been observed in the alloys with bismuth or antimony additions. Both Mg 3 Bi 2 and Mg 3 Sb 2 have a high thermal stability and play important roles in improving creep resistance of the alloys at elevated temperatures.