The purpose of the present study was to investigate the influence of ibuprofen, indomethacin, diclofenac and naproxen for sustained release drug delivery systems, on the thermal and mechanical properties of polymeric free films of Eudragit RS. Free films with different drug contents were prepared by casting-solvent evaporation method. Thermal properties of free films were determined using a differential scanning calorimeter. Morphology of films, mechanical tests and water vapor transmission were also studied. Overall results demonstrated that percent elongation and elasticity of ERS films was increased by incorporation of ibuprofen and indomethacin in the films. Plasticization effect of ibuprofen was mainly due to lipophilic interaction of drug and polymer backbone while in the case of indomethacin, separation of polymeric chains via drug crystallization between polymer segments could affect the elasticity of free films. Naproxen and diclofenac not only had no plasticizing effect but also decrease the flexibility of polymeric films.