Nano cobalt ferrite oxide powders were prepared by sol-gel method. TG/DTG were applied to investigate thermo-decomposition process of the precursor and the results showed that nano cobalt ferrite oxide had different activation energy in every thermo-decomposition stage. XRD and IR were used to analyze the effect of different calcination temperature on phase composition of the precursor. By the analysis of laser granulometer and TEM, the products showed to be nanoparticles of average size within 100 nm. Determined by vibration sample magnetometer, the products’ coercive force and saturation magnetization were 78 kA·m-1 and 72 A·m2·kg-1 respectively.