Physical and chemical indicators of diethylhexyl sebacate (DHS) base oil reacted in different temperatures and conditions were investigated.The thermal oxidative degradation rule of ester base oil was studied by using GC/MS to analyze the constitution changes of the oil samples after thermal oxidation,and the reasons of the degradations on color,viscosity and acid value of ester base oil was revealed on molecular level.The results show that air,oxygen and Nphenylalphanaphthylamine (NPAN) have little influence on the high temperature degradation of viscosity,because the decomposition and the polymerization occurred at the same time result in the decrease and increase in the viscosity respectively.Many oxygencompounds,such as alcohols,aldehydes and organic acids,are generated in oxygen circumstances,which contribute to the increase of acid value of ester base oil.While,NPAN inhibits the increase of acid value of ester base oil effectively and raises the mutation temperature of acid value,exhibiting excellent thermal oxidation resistance.