Abstract: Co_1-xMg _xFe₂O₄/SiO₂(x=0, 0.2, 0.4, 0.6, 0.8) nanocomposite films were prepared by the sol-gel spin-coating process. The effect of the addition of Mg²⁺ with various content on the structure and magnetic properties of the samples was investigated. The structure, morphology and magnetic properties of the films had been studied through XRD, SEM, atomic force microscopy (AFM) and vibrating sample magnetometer (VSM). The results show that the Co_1-xMg _xFe₂O₄ presents spinel structure in composite films with the grain size of about 38~46 nm. As the content of Mg increases, the lattice constant of Co_1-xMg _xFe₂O₄ decreases, the saturation magnetization of sample films also decreases, while the coercivity first increases and then decreases. The value of coercivity and remanence ratio in the direction of parallel to the plane for Co_0.4Mg_0.6Fe₂O₄/SiO₂ sample film is 279.4 kA·m^-1 and 53.9%, respectively, while the corresponding value in the direction of perpendicular to the plane is 350.7 kA·m^-1 and 67.2%. The Co_1-xMg _xFe₂O₄/SiO₂ nanocomposite films display distinct perpendicular magnetic anisotropy.