We study the postbuckling behavior of inhomogeneous thin plates subjected to in-plane compression. We focus on the effect of the inhomogeneity of Poisson's ratio on the nonlinear behavior of the plate in which the buckling modes is kept the translational symmetry. First we discuss the symmetry of the stress function in order to clarify whether the governing equations are solvable as a linear problem. Next we proceed to the nonlinear analysis and derive the postbuckling deflection, its amplitude and resultant forces in explicit forms. Under given concrete expressions of the elastic stiffness, the effects of the material inhomogeneity and orthotropy on the postbuckling deflection, its amplitude and resultant forces are examined.