Abstract: This paper derives tensor formula of elastoplastic stiffness for orthogonal anisotropic materials based on Hill's yield criterion and gives the corresponding explicit expressions for elasto-plastic stiffness matrices in 2-D and 3-D cases, The form of the obtained matrices are quite similar to that for isotropic materials. Therefore, when using F E M to. solve Problems, an existing program for isotropic materials can easily. be modified to include orthogonal anisotropic analysis. As an example, a general purpose nonlinear F E M program is chosen and a material library model is added and, finally, a 3-D cantilever beam is evaluated.