In this research the Mg AZ31 B alloy was submerge friction stir processed (SFSP) and the effects of pin rotation speed (?) and traverse speed (V) on microstructure, texture and abnormal grain growth (AGG) were investigated. Also variations of grain size and hardness of stir zone were identified. In addition, SFSP was applied to incorporate nano-sized Al 2 O 3 into an AZ31 magnesium alloy matrix to form composite surface layer. Samples were subjected to SFSP with and without Al 2 O 3 particles. Microstructural observations were carried out by employing optical and scanning electron microscopy (SEM) on the cross sections both parallel and perpendicular to the tool traverse direction. In addition, X-ray diffraction was used to determined grain orientation distribution. The relationship between the resulting grain size, the applied working strain rate and temperature for submerge friction stir processing in AZ31 B was also examined systematically. Results showed that ultra fine grain (UFG) microstructures with an average grain size less than 1µm could be achieved in a solid solution of hardened AZ31 B magnesium alloy. The mean hardness of the stir zone reached about 95 HV, which is more than twice as high as that of the base metal. When rotation speed decreased or traverse speed increased, texture strengthened and abnormal grain growth increased. Microstructural observations showed that reasonably uniform distribution of reinforcement lead to significant grain refinement of AZ31 magnesium alloy matrix and effectively restricted the grain growth at elevate temperatures such as 400 ? C. Mechanical characterization revealed that addition of nano-Al 2 O 3 particles as reinforcement improves microhardness and wear resistance of magnesium alloy. Results obtained from electrochemical corrosion tests revealed that presence of Al 2 O 3 in the surface of AZ31 B alloy increased the corrosion rate in 3.5% NaCl solution. Key Words Magnesium alloy, SFSP, AGG, nano composite, UFG, Microstructure