Abstract: The heat of oxygen reaction of low rank coals during low temperature oxidation under dry oxygen is measured using pulse calorimeter. The relation between the condensation heat and temperature at low temperature is investigated under wet nitrogen with relative humidity of 80% in order to study the effect of condensation heat on the low temperature oxidation of low rank coal. The results indicate that the heat of oxygen reaction increase with temperature under dry oxygen, but condensation heat decrease with temperature under wet nitrogen. Both kinetic equation of Arrhenius and activation energy are obtained under dry oxygen. It is also found that the condensation heat is much higher than the heat of oxygen reaction when the range of temperature is from 26℃ to 60℃. Therefore, the condensation heat is a key factor affecting the reaction of oxidation of low rank coal at low temperature.