Objective To study the grain refinement behavior and mechanism of AZ91D magnesium alloys after surface mechanical attrition treatment. Methods The AZ91D magnesium alloy in hexagonal-closet-packet structure was treated by surface mechanical attrition treatment ( SMAT) . And the microstructure of surface deformed layer was characterized. Results After 60 min of SMAT, the depth of deformed surface layer was about 80 μm, the structure of which showed a gradient distribution, and the grains became larger with increasing distance from surface, and the surface grain size reached about 20 nm. Conclusion The grain of AZ91D magnesium alloys was obviously refined by surface mechanical attrition treatment; the grain refinement mechanism of AZ91D magnesium alloy by surface mechanical attrition treatment was the results of twinning and dislocation slip.