In this study, the pure polystyrenes (PS) with different molecular weights (3.5 x 10(5) and 5.0 x 10(5)) have been modified by the chemical modification with succinic anhydride (SA), maleic anhydride (MA), and phthalic anhydride (PhA). The modified polystyrenes (MPS) have been mixed with the pure PS with the molecular weight of 2.3 x 10(5) in weight % ratio 90:10, 80:20, and 70:30. Young's modulus of obtained composites has been measured mechanically by the tensile test and ultrasonic method at frequency of 5 MHz. Further, the values of Young's modulus measured by both methods have been compared with each other. From the results, a significant difference has not been found between the values of Young's modulus of both methods. As a result it can be stated that measuring the Young's modulus of these materials by the ultrasonic methods is more sensitive and economical than the mechanical methods.