Since coal is a mixture of various aromatic rings, aliphatic chains, and functional groups, its fluidity could not be understood from a chemical point of view. We represented the average formula of coal by CHmOn and succeeded in estimating the enthalpy of formation of coal. It is noteworthy that m of CHm On is determined by the numbers of H and C atoms in aromatic ring and side chain of coal. We prepared a table that describe the values of H/C of carbon skeletons consist of various aromatic rings and side chains systematically. We searched for a value of H/C closest to m in this table and presume the number aromatic rings and the form side chain, followed by estimation of carbon skeleton model of the coal, and studied the relationship between the fluidity of coal and its structure of aromatic ring.