Chain flexibility and mechanical properties of four polyimides with different chemical structures are simulated by molecular dynamics and molecular mechanics techniques to establish some structure-property relationships. The oxygen linkage in the diamine moiety of a polyimide gives the highest flexibility whereas the sulfonyl linkage imparts the lowest flexibility to the polymer chain. A more flexible polyimide has smaller characteristic ratio, lower solubility parameter, lower elastic modulus, and larger yield strain. These simulated values show good agreement with experimental data.