We have developed an evaluating method of plastic Poisson's ratio as a function of equivalent plastic strain (PS) for polymer materials. The PS is used to simulate permanent volumetric deformation due to void extension and plays an important role in simulating mechanical properties of crazing behavior which appears in uniaxial or biaxial tension state. Unfortunately stress-strain relation (SS) strongly depends on PS, which makes the evaluations of PS and SS data difficult. In this paper we have proposed a self-consistent equation of true stress for uniaxial / biaxial stress state. We can successfully separate the evaluations of PS and SS by introducing the equation into the processes, and the problem has been solved. Hence this method enables us to build an application that automatically evaluates PS and SS with some optimization techniques such as GA.