Most of sheet-metal forming simulations assume that the anisotropy of a sheet is fixed constant throughout the forming process. However, some materials show apparent anisotropy evolution with increasing plastic deformation. This paper presents a framework of description of anisotropy evolution, where an anisotropic yield function, which varies continuously varies with plastic strain, is defined as an interpolation between two yield functions at two discrete levels of plastic strain. The effect of anisotropy evolution was examined for several types of sheet metal forming processes, such as hydraulic bulging, conical-cup drawing, and cruciform-shaped drawing, by performing these experiments on a mild steel and type-5000 aluminum sheets together with the corresponding finite element (FE) simulations. From the comparison of FE simulations with the experimental results, it was concluded that the description of anisotropy evolution is essential for accurate simulation.