The thermoplastic characteristics of a packed bed of coal with different particle sizes were investigated in carbonization. The dilatation of the packed bed of coal was examined using a dilatometer, and the amount of volatile matter released from the coal particles was evaluated using a thermogravimetric meter. As a result, the total dilatation (TD) of a coal bed with an average diameter of 67 µm was 127%, and that with an average diameter of 60.5 µm was 80%. We revealed that its particle size affects the swelling of the packed bed of coal. The release behavior of volatile matter (i.e., temperature and the amount of the releases) was different depending on the particle size. Therefore, the release rate and amount of volatile matter depend on the coal particle sizes, and the particle size of coal particles affects the swelling behavior of the packed bed of coal.