Abstract: Liaohe heavy oil was heated for 2 hours in an autoclave at 350℃, 375℃ and 400℃, respectively. Saturates, aromatics, resins and asphaltenes were separated from oil sample. Elemental analysis, VPO molecular weight determination, IR analysis and H-NMR analysis for asphaltenes and resins were conducted. The results show that as increasing reaction temperature, the ratio of (asphaltenes and saturates) / (resins and aromatics) increases, the stability of the colloidal system of the heavy oil decreases. After thermal reaction, the molecular skeleton structure of resins and asphaltenes is not basically changed, but the breakage of side chain causes a reduction of molecular weight. Above 375℃, as the side chain is completely broken out, the degree of association of the asphaltenes enhances, thus its molecular weight increases.