Conformational elasticity theory recently developed has been used to explore the internal energy contribution to the elastic force fe/f as a function of strain for polyethylene (PE). Calculated fe/f values are in good agreement with those obtained experimentally. Results show that the behavior of fe/f is mainly contributed from intramolecular interaction of chemical structure, and that the internal energy contribution is strain-dependent. Conformational elasticity theory recently developed has been used to explore the internal energy contribution to the elastic force fe / f as a function of strain for polyethylene (PE). Calculated fe / f values ​​are in good agreement with those obtained experimentally. Results show that the behavior of fe / f is mainly contributed from intramolecular interaction of chemical structure, and that the internal energy contribution is strain-dependent.