dBowing distortion phenomena of bi-axially oriented film cause serious problems in practical application. Bowing distortion was conveniently defined to carry out the analysis. Bowing distortion profile throughout a tenter machine was observed. This profile was fairly successfully simulated by FEM using suitable anisotropic and heterogeneous elastic constant distributions, and a suitable initial shape of the elastic body. The mechanism of bowing distortion in relation with TD stretching was estimated by the above results and some experimental data. This leads to the forecast that the insertion of a cooling zone between the TD stretching zone and the thermosetting zone will be an effective way for decreasing bowing distortion of film. This forecast was experimentally verified. Despite the insertion of the cooling zone, a certain level of bowing distortion still remained It was tried to give an explanation for the mechanism of the remaining distortion in terms of heat-shrinkage force in the thermosetting zone.