A nanocrystalline layer(NL) was fabricated on the surface of AZ31 magnesium(Mg) alloy sheet by surface mechanical attrition treatment(SMAT). The microstructure of the Mg alloy was characterized by optical microscopy,X-ray diffraction and microhardness test. The results showed that both the microstructure and microhardness of AZ31 Mg alloy sheet after SMAT revealed a gradient distribution along depth from surface to center. The thermal stability of the NL was investigated through characterizing the microstructure evolution during the post-isothermal annealing treatment within the temperature range from 150 to 250 ℃. The NL exhibits a certain degree of thermal stability below 150 ℃, while it disappears quickly when annealing at the temperature range of 200–250 ℃. The grain growth kinetics of the nanocrystalline of AZ31 Mg alloy induced by SMAT was investigated. The activation energy of nanocrystalline AZ31 Mg alloy was obtained with a value of 92.8 k J/mol. The microstructure of the Mg alloy was characterized by optical microscopy, X-ray diffraction and microhardness test. The results show that the nanocrystalline layer (NL) was fabricated on the surface of AZ31 magnesium (Mg) alloy sheet by surface mechanical attrition treatment showed that both that microstructure and microhardness of AZ31 Mg alloy sheet after SMAT revealed a gradient distribution along depth from surface to center. The thermal stability of the NL was investigated through characterizing the microstructure evolution during the post-isothermal annealing treatment within the temperature range from 150 to 250 ° C. The NL exhibits a certain degree of thermal stability below 150 ° C. while it disappears quickly when annealing at the temperature range of 200-250 ° C. The grain growth kinetics of the nanocrystalline of AZ31 Mg alloy induced by SMAT was investigated The activation energy of nanocrystalline AZ31 Mg alloy was obtained with a value of 92.8 kJ / mol.