In order to investigate coal plasticity, thermogravimetric analyses (TGA) of 18 kinds of coking coal were carried out. When coals are heated under an inert atmosphere, weight loss appears at around 400-500 degrees C. We found that the maximum rates of weight loss normalized by the amount of total weight loss up to 1000 degrees C showed a good correlation with Gieseler maximum fluidity of the sample coals. This result means that effective generation of tar governs the degree of fluidity of coal, supporting strongly the contribution of low-molecular components, metaplast, to the plastic behavior and fluidity of coking coals during the carbonization process. In addition to this, the analyses of tar or gaseous products evolved during heat treatment were also conducted in order to compare structural differences of these fractions among a series of coking coals. The effect of heating rate on TGA of the sample coals was also investigated.