Using a simulated laminated-plate model, an analytical method is presented for predicting stiffness of fiber reinforced injection moldings including the effects of fiber orientation and fiber length distributions. The mechanical properties of fiber reinforced composites depend not only on the elastic properties and the volume fraction of the constituent materials, bur also on the fiber orientation and fiber length distributions in the molding. In this study, two probability density functions are proposed for modeling the distributions of fiber orientation and fiber length in the injection moldings, respectively. Inserting the fiber orientation and fiber length distributions into the components of the stiffness constant, the elastic moduli of the fiber reinforced injection moldings are predicted on the basis of a generalized laminated-plate theory.