Fuel deposition (FD) as an important stage in the oxidation process of the crude oil in the air injection technique was less studied in detail because the FD was not obvious in the thermogravimetrydifferential thermogravimetry (TGDTG) experiments in previous studies. In this study, the obvious FD in TGDTG curves and coke formation in the FD process in the isothermal oxidation experiments of the heavy oil were observed. The coke formation and FD characterization of the heavy oil were further investigated using isothermal oxidation experiments and TG-DTG experiments, respectively, by making a comparison to light oil. These isothermal oxidation experiments were carried out at 120 degrees C and 30-40 MPa. Gas chromatography was employed to analyze the composition of C-1-C-6, O-2, CO, and CO2. The FD characterization was analyzed by TGDTG curves. The Arrhenius method (linear regression) was used to obtain the kinetic parameters. In the isothermal oxidation experiments of the heavy oil, the coke was formed. Both the formed coke and the oxidized oil were flammable under ambient temperature. However, there is no coke being formed for the light oil. On the basis of the calculated kinetic parameters, a new understanding of the FD was proposed. The whole FD process could be divided into two subzones: positive temperature coefficient (PTC) zone, where the activation energy was positive, and negative temperature coefficient (NTC) zone, where the activation energy was negative. For the heavy oil, both NTC and PTC are obvious and the detritus could clearly extend the temperature range of the FD, especially for the PTC subzone. However, for the light oil, the FD mostly showed the NTC subzones. Only a small PTC subzone was displayed.