In this investigation, the effect of Polyvinylpyrrolidone (PVP) additive on microstructure, morphology and magnetic properties of cobalt ferrite nanoparticles prepared by hydrothermal method was studied. X-ray diffraction (XRD) studies in different synthesis conditions showed the formation of cobalt ferrite and cobalt oxide. Comparing IR spectrum of PVP additive, sol prepared before hydrothermal process and C-0.1PVP3, 190 obtained by FTIR spectroscopy indicated the formation of bond between PVP and surface of metallic hydroxide and cobalt ferrite particles, which prevented them from growing and coarsening. Scanning electron microscope (SEM) was used to study the morphology of samples. According to vibration sample magnetometer (VSM) results, as PVP amount increases from 0.1 to 0.3 volume percent, coercive field increases from 298 to 684 Oe and saturation magnetization decreases from 58 to 51 emu/g.