The low temperature oxidation of four coals ranging from brown coal to bituminous coal was investigated. The coal oxidation was carried out using O-16(2) or O-18(2). Isotope O-18(2) was used for the oxidation experiment of coal to clarify the release behavior of oxygen-containing gases during the oxidation of the coal sample and the devolatilization of oxidized coal. During the oxidation of coal and the temperature-programmed heat-treatment of oxidized coal, CO (m/z = 28, 30), CO2 (m/z = 44,46,48) and H2O (m/z = 18, 20) were analyzed with a quadrupole mass spectrometer. It was found that the gas release profiles were dependent on the coal rank. In the case of high rank coal, the amounts of (H2O)-O-18, (CO)-O-18 and (COO)-O-18-O-18 released than those of (H2O)-O-16 (CO)-O-16 and (COO)-O-16-O-16. Oxygen chemisorped in low rank coal is easily released as inorganic gases during the heat treatment. The oxygen incorporated into the coal structure during the oxidation treatment plays more important role for the decrease in the caking property than oxygen in raw coal. The coal structural changes, which may be due to cross-linking and the decrease in hydrogen donor ability of oxidized coal, strongly affected the caking property of coal.