Co2+-doped Fe3O4 magnetic nanoparticles were synthesized via the solvothermal method with reaction system of H2O and glycol in a high pressure autoclave. The products are of the inverse spinel structure confirmed by X-ray Diffraction. Fourier Transform Infrared Spectroscopy, Transmission Electron Microscope, Particles Size Analyzer and AC Gradient Magnetometer are also used to characterize the particles. The obtained Co2+-doped Fe3O4 magnetic nanoparticles display well crystalline state. Those particles are globular of which the diameter is above 15 nm with homogeneous size distribution, smaller than 20 nm of the non-doped products. Co2+-doped Fe3O4 magnetic nanoparticles show superparamagnetic behavior, and the saturation magnetization is 76.84 emu/g, which is higher compared with 54.42 emu/g of the non-doped. In the crystals structure of Fe3O4 magnetic nanoparticles, ferric ions occupy the tetrahedral sites and one-half of octahedral sites, and the ferrous ions occupy the remaining half of the octahedral sites. The magnetic moments of cations on tetrahedral and octahedral sites are anti-parallel. Therefore, the net moment is provided by the divalent ions. In the preparation process, bivalent cobalt ions replace the partial sites of the ferrous ions and the ferric ions, and enhance the net magnetic moment. Above all, the doping of Co2+ could improve properties of the Fe3O4 magnetic nanoparticles, i.e., enhancing the saturation intensity, decreasing the particle size and making the size distribution homogeneous.