Abstract: The carbon nanotubes (CNTs) reinforced magnesium matrix composites were fabricated by adding 0-1.5wt% CNTs into the molten Mg-1.3Mn-1.0Ce-4.0Zn alloy using stirring casting process. Microstructure and mechanical properties of the composites were studied and toughening mechanism of the composites was also discussed. The results show that grain size of the substrate alloy was refined by CNTs. Meanwhile, the grain morphology and distribution characteristics of second phase also were changed by CNTs. With the increase of CNTs′ adding contents, the room temperature strength, fracture elongation and hardness all increased firstly and then decreased. When the mass fraction of CNTs was 0.5%, the composites exhibited the highest tensile strength, fracture elongation and Brinell hardness, they were 212.2 MPa, 21.1% and 55.0 HBW, respectively, they respectively increased 8.5%, 37.5%, 10% comparing with the matrix alloy. The strengthening mechanism of the composites was reinforcement phase strengthening, second phase strengthening and fine-grain strengthening. Meanwhile, grain refinement, CNTs lubrication and CNTs retarding crack extension were mainly contributed to the improvement on plasticity.