A warpage index (Delta psi(m)) was introduced for studying warpage characteristics of a plastic part injection molded from PA66 compounded with 30 wt% glass fiber. Delta psi(m) is defined as Delta psi(m) = (psi(m))(max) - (psi(m))(min), where psi(m) = psi(theta)(max) where psi(theta) = (epsilon(theta) - alpha(theta)Delta T)/ alpha(theta)Delta T , where epsilon is the total strain, alpha is the linear thermal expansion coefficient, Delta T is temperature difference, and theta is the angle along which epsilon and alpha are calculated. Finite element analysis was used for calculating flow field in injection, fiber orientation, material anisotropy and warpage. psi(m) is calculated in each finite element, and Delta psi(m) is calculated in a whole finite element model. Delta psi(m) is a measure of the ratio of actual shrinkage to the amount of shrinkage that would occur if an element freely shrank. The characteristics of Delta psi(m) were studied. It has been found that warpage is null if Delta psi(m) = 0, but that null. warpage generally does not indicate Delta psi(m), = 0 It is shown that Delta psi(m) quantitatively represents the warped and unwarped state. Delta psi(m) distinguishes the null warpage state with possible buckling from the null warpage state without possible buckling. It has been shown that material anisotropy is possibly described with Delta psi(m), and that the cause of warpage is self-restrictive deformation in an injection molded part. It has been deduced that it is generally not possible to eliminate warpage only by controlling material properties. Delta psi(m) is obtainable for a plastic part with complex geometry and complex fiber orientation state, and for arbitrary materials. Applications of Delta psi(m) are left for future study.