In this work, the thermal behaviors, kinetic parameters, high pressure properties, and product characteristics of Qi 40 heavy oil in air, nitrogen, carbon dioxide, oxygen-denuded air, and flue gas atmospheres were systematically and comparatively studied by thermogravimetric analysis, kinetic analysis, pressure-volume-temperature test, and stimulation experiments. The thermal decomposition of heavy oil occurred in three major zones. The comprehensive devolatilization indices of heavy oil in nitrogen, carbon dioxide, oxygen-denuded air, air, and flue gas atmospheres were 3.32 x 10(-7), 2.94 x 10(-7), 2.07 x 10(-7), 2.30 x 10(-7), and 2.58 x 10(-7) % K-3 min(-2), respectively. The kinetics analysis indicated that, except for zone 2 in oxygen-denuded air, the best kinetic models of three zones were (1 - x)(1/3), (1 - x)(3/4), and (1 - x), respectively. Pressure volume-temperature tests displayed that the high pressure properties of heavy oil were mainly influenced by the gas solubility effects at low temperatures, and the heating effects at high temperatures. Moreover, Fourier transform infrared spectra, crude oil group, element, and gas chromatography analysis on products from stimulation experiments of heavy oil all showed that the product characteristics might be influenced by the existence of oxygen in the non-condensable gases. The results of this study provided useful information for potential application of non-condensable gases injection on thermal enhanced oil recovery.