The relationships between the chemical structure, packing density( 1/V-F), and cohesive energy density (CED) and the thermal properties of polyimides were investigated. Particularly, the correlation of tan delta measured by stress-strain/thermal mechanical analysis with 1/V-F and CED was found for eight polyimides. We measured the relationship between the apparent diffusion coefficient (D-a) and 1/V-F and CED, respectively, as described in previous articles. From these experiments, we found that the thermal properties, especially tan delta, were correlated with the apparent diffusion coefficient of gas. These results are well explained by use of micro-Brownian motion.