A plastic constitutive model for initially orthotropic but subsequently skewed anisotropic materials is proposed in this paper. The variation of anisotropic axes is discussed within the framework of objectivity, and the evolution of the axes is formulated. It is shown that the yield function proposed is identical to Hill's quadratic yield function when the anisotropic axes lie along the base vectors. Applying the conventional laws for both work-hardening and material flow to the yield function, an anisotropic constitutive model is completed. A finite element equation is obtained next, based on the updated Lagrangian formulation. A fully three-dimensional analysis is carried out to predict the strain localization behavior of anisotropic sheet metals. The direction of strain localization is strongly affected by the initial configuration of the anisotropic axes, and the simulated result shows fairly good agreement with the experimental one.