Four low rank coals (peat and lignite) and their derived humic acids have been investigated by the C-13 CP/MAS/TOSS NMR technique. The NMR analysis indicates that the peat contains a significant amount of unaltered plant components including cellulose, hemicellulose, lignin, waxes, and resins. Variable contact time and dipolar dephasing experiments were performed on each sample. The data from above experiments, together with normal CP/MAS integration over selected chemical shift range, were used to derive 12 parameters of carbon skeletal structure including the aromaticity. From the ratio of aromatic bridgehead carbons, f(a)B, to the aromatic peripheral carbons, f(a)CP, x(BP), as measured by NMR, the aromatic cluster size may be estimated for each coal.