A novel orthogonal microscope image analysis method was developed to observe and evaluate quantitatively the dynamic solvent-swelling behavior;Ind characteristics of coal particles separated using density gradient centrifugation technique. The important results obtained using the new method are as follows : (1) The maximum statistic probability of anisotropic swelling ratio from 1.1 to 1.3 measured in this work agrees very well with the data measured by Cody et al.; in addition, the samples with high vitrinite concentrate and high swelling ratios seem to have high anisotropic swelling feature. (2) The existence of a characteristic distribution of volumetric swelling ratio in the vitrinite concentrates strongly suggests the existence of a characteristic distribution of the molecular weight between cross-link points. (3) The average volumetric swelling ratios measured using the new method are comparable to that measured by the traditional packed bed method. (4) The simulated results using modified Painter＇s disinterspersion model based on the measured characteristic swelling ratios suggest that each cluster has aromatic and hydroaromatic rings from 1 to 3 in PDC-HV coal and from 1 to 5 in Witbank coal. Moreover, the relatively short average virtual bond length per carbon atom and the random walk of the chains are the main features of the coals in pyridine. (5) Although the case II and super-case II are dominant as for diffusion of pyridine in the coal particles, a few cases of Fickian and anomalous diffusion are also existing for two kinds of coal. In addition, because the value of diffusion exponents, n, of some PDC-HV coal samples changes from 0.22 to 0.39, which does not match to any classical diffusion mechanism, it seems to be the reflection of one diffusion mechanism.