Coal is the third source of energy and its reserve is abundant. Two major problems of using coals as an energy source are (i) the cost of transportation and (ii) the emission from coal-fired power generator. To overcome the emission problem, research in clean coal technology that includes mild gasification has been done. This paper presents our experimental results of reactivity studies of coal combustion as a part of research for clean coal technology. Using the TGA (thermogravimetric analyzer) in our combustion laboratory, three types of Malaysian coals are investigated in this work: Silantek, Mukah-Balingian, and Merit Pilla. The combustion parameters used are combustion temperature, oxygen atmosphere, combustion heating rate, and fluid velocity. The combustion processes are performed under mild gasification conditions. Various particle sizes of the coals are studied systematically. Proximate analysis shows that the Silantek coal contains the highest fix carbon while the Mukah-Balingian coal contains the lowest fix carbon. Silantek coal has been shown to have the highest calorific value of 7317 cal/gram. The self-ignition phenomenon of each type of coals is discussed. It can be concluded that the likelihood of ignition increases with combustion heating rate.