Recently, a new simple test method called plane strain stretching(PSS) test has been developed and used successfully in the evaluation of the press formability of automotive steel sheets. In this paper the tool geometry of the PSS test were optimized in order to assure the reliability of the test. Considering many factors for optimization of the tool geometry, computer-simulation technique using three-dimensional finite element method(FEM) was used. Three design variables - the punch length, punch crown and punch corner radius - are chosen to be optimized according to the Taguchi's experiment technique with the L9 orthogonal array. The optimum condition to ensure the plane strain mode over the overall area of the specimen was clarified. Moreover the simulation results are confirmed by experiment.From this simulative study, we can conclude that the optimized punch shape of the PSS test with ellipsoidal shape shows a stable performance in evaluating stamping formability of sheet materials. Strictly speaking, the optimized punch geometry with the punch crown 3mm, punch corner radius 6mm and punch length 72mm shows a better reliability compared with that of the original PSS test.