In this paper, the effect of multi-pass friction stir processing on the mechanical properties of AZ91 alloy has beenwas studied. This process haswas been simulated with the three-dimensional model. This numerical model is based on the ABAQUS/Explicit. This simulation involves the Johnson-Cook modelsmodel for defining the material behavior during thisthe intense plastic deformation and investigating the fracture criterion. Here the tool plunging and stirring phases in the two-pass process haswas been simulated. To prevent the distortion of elements during modeling, the Arbitrary Lagrangian-Eulerian technique for automatically re-meshing of distorted elements has beenwas used. The model was calibrated using the experimental results from the previous works. Using this model, the transient temperature distribution and residual stress field during FSP on AZ91 can be predicted. The results show that the maximum temperature in the advancing-side region is more than that in the retreating-side region. In addition, numerical results show that at the end of the process, the tool during the lift-up leaves the keyhole region in the compressive stress state. Also, the experimental results investigated the effect of multi-pass FSP on the microstructure, microhardness, tensile and creep Strength of AZ91 magnesium alloy. The tensile, and creep tests arewere conducted at several temperatures, from 25 to 210 °C. The optical microscopy and scanning electron micrograph (SEM) were used to study the microstructure of the samples after performing multi-pass friction stir processing. The experimental results indicate that, on average, the tensile strength, microhardness, and creep strength of the processed samples increased 29, 23, and 38%, respectively compared to the unprocessed onessamples.