Co_xNi_1-xFe₂O₄ (x=0~1) nanoparticles were synthesized by hydrothermal method. And then X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and magnetic scanning magnetometer were used to characterize the structural, morphological and magnetic properties. XRD results indicate the lattice constant of Co_xMn_1-xFe₂O₄ decreases with increasing Co²⁺ substitution. Magnetic measurements show that all the samples are ferromagnetic at room temperature, and magnetization and coercivities of the samples increase with Co²⁺substitution.