Abstract: Constant stress tensile creep tests were conducted on the 25vol% Al₂O₃SiO₂ short fiber/AZ91D composite and the unreinforced AZ91D matrix alloy at 473K and 573K under the applied stresses of 30~100MPa. The true stress exponent, the true creep activation energy, the threshold stress and the creep constitutive equation of the composite were calulated according to the curves of strain and strain rate. Microstructural investigations by TEM reveal that the threshold stress of the composite results from the pinning effect of MgO particles and Mg₁₇Al₁₂ precipitation on the fiber surface on the movable dislocations during creep exposure. Short fibers have a great function in load bearing and load transfer, and result in the enhancement of the creep resistance of the composite.