Cf/Mg composites with three kinds of carbon fiber （without coating, coated with Ni or SiO2） were fabricated by vacuum pressing method. The microstructure and fracture surface were characterized by SEM, EDS and TEM. The results show that the wettability between carbon fiber without coatings and Mg was low. And the wettability is enhanced by coating the carbon fiber with Ni or SiO2. The Ni coating reacting with Mg obtains Mg2Ni. The fiat fracture of the Mg matrix composite reinforced by carbon fiber with Ni coating shows that the combination of the carbon fiber with Mg matrix is very strong. The fracture surface of the Mg matrix composite reinforced by carbon fiber with SiO2 coating shows that the combination of the interface is optimum.