Abstract: Through investigating the influence of various process parameters on the hydrotreating reactions of FCC recycle oil, the variations of various types of hydrocarbons, S and N contents in hydrotreated recycle oil with process parameters were obtained. Results showed that the mass fraction of paraffin and naphthenic hydrocarbons changed a little with reaction conditions. With the increase of H2/oil ratio, reaction time and hydrogen partial pressure, the reactions of HDA, HDS and HDN were benefited, yet when the H2/oil ratio was over an optimal value, the effect was little. The increase of reaction temperature benefited the proceeding of HDS reactions; due to the limit of thermodynamic equilibrium for HDA and HDN reactions, there was an optimum reaction temperature for each respectively, and the temperature of the latter was higher than that of the former. Under the same hydrotreating conditions, polycyclic aromatic hydrocarbons having various structures exhibited various hydro-saturation activities, e.g. the hydrogenation of peri-condensed polyaromatic hydrocarbon was more difficult than that of cata-condensed polyaromatic hydrocarbon.