The effect of ultrasonic vibrations on metals which appears as a decrease in yield strength and is known by the acoustic softening, has been an interest of researchers for a long-time, is studied in this thesis. Understanding the cause of this phenomenon and its affecting factors is highly important. In this study, after reviewing previous researches, acoustic softening factors was investigated and by using the experiment design techniques of Taguchi, simultaneous effects of four parameters, namely, Stacking-Fault Energy (SFE), Vibration Amplitude, Tensile Test Speed and Duration of applied vibration were studied. In this study to observe effect of stacking fault energy, 4 different alloys were used: brass alloy, copper with high purity, Aluminum 2014 and Aluminum 6033 alloys to study a wide range of stacking fault energy. Frequency of vibrations in all experiments was 20 kHz. Based on the Taguchi experiment design, an orthogonal array with 32 rows, which suggests 32 tests to obtain the best answer, was selected. Samples were prepared by the following procedure: quantometry of raw materials, release of residual stresses caused by the extrusion process in the production of round billets and design and preparation of standard samples. Ultrasonic components to apply ultrasonic vibrations in simple tensile test including horn and sample clamp were designed by the finite element simulations. The results have been evaluated by Artificial Neural Network (ANN) and a multi-layer network for predicting the amount of acoustic softening of metals with FCC structure was produced. Using the results generated by the network, considered factors were investigated, and it was revealed that the vibration amplitude and sample material were more effective and then strain rate and time period of imposing vibrations had less effect on softening. The amount of softening increased with increasing the vibration amplitude, reduction of tensile test speed and generally reduction of time period. The highest amount of softening happens in Aluminum 2014 alloy. On the other hand, to investigate the cause of softening phenomenon under ultrasonic vibrations, the vibrated and non-vibrated samples were compared using X-ray diffraction, and the results of the change in density of dislocations caused by ultrasonic vibration have been reported. Densities of dislocations were reduced in materials exposed to ultrasonic vibration which is believed to be the cause of the softening phenomenon. Keywords Ultrasonic vibrations, acoustic softening, Design of experiments, Taguchi method, orthogonal arrays, Artificial Neural Network (ANN), multi-layer network, X-Ray diffraction, density of dislocations.