Abstract: Hydroxyapatite (HA) reinforced WE43 magnesium composites (HA/WE43) were prepared by friction stir processing (FSP), the influence of rotation speed on HA distribution and the changes of microstructure and mechanical properties before and after FSP were studied. The microstructure of HA/WE43 composites was characterized by optical microscope, SEM and TEM, and the microhardness and tensile properties tests were conducted to evaluate the mechanical properties. The results show that the grain size of HA/WE43 composites is significantly finer than that of the base materials. During the processing, the presence of HA particles enhances the grain refinement effect of FSP. HA particles tend to cluster in the HA/WE43 composites prepared at a relatively low rotation speed. As the rotation speed increases, the HA particles distribute more uniform and the agglomeration phenomenon is improved. Although locally agglomerated HA particles can become the origin of cracks in the composite during tensile deformation, an improvement of ultimate tensile strength, yield strength and elongation is achieved in the HA/WE43 composites.