In this study, an optical method is developed for the determination of the mechanical properties of polymers. The proposed method utilized the transmitted and reflected caustics. Thus, it can be used in a typical optical stress analysis laboratory. The mechanical properties of an optically isotropic material are determined using thin specimens with a central hole that are loaded in tension. The diffraction effects were considered using a digital image-analysis system. The effect of the hole size on the determination of the mechanical property is investigated. Excellent results were obtained using the proposed method.