Work behaviours and training effects of artificial muscle based on electrochemomechanical strain (ECMS) of conducting polymer, polypyrrole (PPy) films have been studied under tensile stress up to 5 MPa. PPy films were electrochemically deposited in dodecylbenzene sulfonic acid. Cyclic voltammetry and ECMS were measured in 1M NaCl aqueous solutions. At higher stress, a larger creeping due to the conformation change, slipping and/or breaking of polymer chains was observed. After the experience of large tensile stress an appreciable increase of strain (training effect) was observed, when the film was actuated at lower tensile stress. The training effect was discussed in terms of the relaxation of anisotropic strain induced during the creeping. The conversion efficiency from electrical to mechanical energies was 0.07 % at 3 - 4 MPa being the maximum. Degradation of film was found to be accelerated under higher tensile stress from the measurement of conductivity.