Warpage of two injection molded parts with similar geometry was investigated to clarify the key parameters for determining the qualitative warpage mode in injection molding. The geometry of one part (Case 1) included small ribs in addition to the geometry of the other part (Case 2). The basic geomerty of Case 1 and Case 2 were plate with three rectangular ribs, and were identical to each other. PET compounded with 30 weight % short glass fiber was used in the study. It was experimentally found that the warpage mode of Case 1 did not qualitatively coincide with that of Case 2. From thermal stress analysis and buckling analysis, it was shown that the difference of the fiber orientation states between Case 1 and Case 2 was not adequate for generating a qualitative difference in warpage mode between the two parts. The numerical analysis indicated that the Case 2 warpage was caused by the anisotropic material properties which were induced from the fiber orientation state. On the other hand, the buckling analysis indicated that the measured Case 1 warpage required an external force applied to the part during the injection molding. The possiblity was shown that external force influences to the warpage modes of injection molded parts. The detail investigations on the existence of the external force and on how the external force affects to the part warpage are left for the future study.