A polypyrrole (PPy) film, prepared electrochemically from methyl benzoate solution of tetra-n-butylammonium tetrafluoroborate on Ti electrode, showed 12.4% strain and 22 MPa stress induced electrochemically in NaPF6 aqueous solution. The large strain and stress may be due to doping-dedoping of anion and a conformational change of PPy chains.