Changes in mass with time due to gas evolution during carbonization were observed for particles and a cylindrical pellet of various coals heated respectively in a thermogravimetric balance and a microautoclave. The experiments were carried out for a range of the heating rate from 1 to 50K/min, the holding temperature from 673 to 823K and the nitrogen gas pressure of 0.1 and 1.0MPa. Effects of these operating variables on the gas evolution characteristics were experimentally examined. It was demonstrated that the gas evolution characteristics depend on the heating rate and the holding temperature as well as coal nature. Moreover, the total amount of gas evolved at a holding temperature was shown to be independent of the heating rate and to increase with the holding temperature. A simple reaction model is proposed for gas evolution which assumes coal to consist of two components having different reactivities for pyrolysis; one producing gas along with formation of semicoke via plastic intermediate and the other converting directly to gas. The model parameters were analyzed by fitting the model to the observed results and successfully utilized to explain observed results at different operating conditions.