An experimental device for determining Young’s modulus of polymer plates is presented. Two sets of polymer blends were investigated. First samples were made of ethylene propylene diene (EPDM), high density polyethylene (HDPE), and meleic anhydride grafting on polyethylene (PE-MA) reinforced with glass fiber. The second set of samples was made of low density polyethylene (LDPE) reinforced with nylon fiber. Young’s modulus of a circular polymeric plate under biaxial stress was determined by measuring its out-of-plane displacement. This test is also known as bulge test. Values of Young’s modulus of the polymer plates were compared to those obtained from standard uniaxial tensile tests. Biaxial stress causes tension in all internal reinforcing fibers of the samples, in contrast tensile tests that cause tension mainly in fibers aligned with the applied force. Out-of-plane displacement was measured applying a laser triangulation setup based on a projected laser line and a CCD camera. A mathematical model for plates under the bulge test was then used in Young’s modulus calculations using out-of-plane displacement and plate dimensions. It is proved that the bulge test is more sensitive to reinforcement fiber compared to the standard uniaxial tensile test.