AZ80 magnesium based composites strengthened by multi-walled carbon nanotubes (MWCNTs) of different volume fractions were fabricated by friction stir processing (FSP) and the effect of heat treatment on microstructure and mechanical property of MWCNTs/AZ80 were investigated. The results show that the Mg-based composites are dense with fine grains, in which MWCNTs distribute uniformly. A13Ni2 intermetallic compounds are formed on the interface between the MWCNTs and the magnesium matrix, leading to a sound bonding condition. The tensile strength of the composites increases with the increase of volume fraction of the MWCNTs, and it is also improved by the post heat treatment of solid solution and aging. The major factor affecting the mechanical property of the MWCNTs/AZ80 is the interface structure between the MWCNTs and the magnesium matrix