Currently, magnesium (Mg) and its alloys are the lightest structural metallic materials. Although the mechanical properties of Mg alloys can be improved by adding alloy elements, it causes a significant reduction of thermal property. Therefore, the application of Mg alloys is limited. Carbon nano-tubes (CNTs) are one of the most ideal reinforcements due to their excellent mechanical and thermal properties. They can be used to improve the mechanical and thermal properties of Mg alloys. Using pure Mg, Mg-9Al and Mg-6Zn as matrixes, the magnesium matrix composites with various CNTs contents were fabricated via the powder metallurgy method. The microstructures , interfaces between CNTs and matrix as well as precipitated phases of composites were characterized by OM, SEM and TEM. Moreover, the mechanical and thermal properties of composites were measured. Results indicate that the thermal conductivities of CNTs/Mg composites are enhanced with CNTs content less than 1.0wt%, while their mechanical properties only have a slight decrease. As CNTs are added into Mg-9Al alloy, a few nano-sized β-Mg17Al12 phases precipitates around CNTs. It has reduced the solid solubility of Al atom in Mg matrix, so that the thermal conductivity of CNTs/Mg-9Al composite increases. In CNTs/Mg6Zn composites, there is an area where C and Mg atoms have embedded in each other at the interface. On the one hand, the embedding area coud improve the mechanical properties of CNTs/Mg-6Zn composite. On the other hand, it could make CNTs as a “bridge” to accelerate the movement of electrons, accordingly increasing the thermal conductivity of composites. When the CNTs content is 0.6wt%, CNTs/Mg-6Zn composite has relatively excellent thermal and mechanical properties. Its thermal conductivity is 127.0 W/(m·K), and its ultimate tensile strength is 303.0 MPa, tensile yield strength is 204.0 MPa and elongation is 5.0%.