Various copolyimides were prepared from two acid dianhydrides (BPDA, 3,3’,4,4’-biphenyl tetracarboxylic dianhydride; PMDA, pyromelitic dianhydride) and two diamines (PPD, p-phenylene diamine; ODA, 4,4’-oxydianiline). The thermal and mechanical properties of these polyimides were examined in detail. By appropriately selecting the ratios of the acid dianhydride component and the diamine component, polyimide films having desirable mechanical and thermal characteristics can be obtained. Further, it was proved that there is a correlation between the properties and the compositions of the copolyimides and that the properties could be estimated from the compositions by the use of multiple regression analysis.