Ultrasonic welding of thermoplastics has become an important process in industry because of its relatively low cost and its high quality joints. However, the optimization of this technique has been essentially based on a trial-and-error process. In this report, an L18 experimental matrix design based on the Taguchi method was conducted to optimize the joint strength of ultrasonically welded thermoplastics. For the factors selected in the main experiments, weld time and amplitude ol: vibration were found to be the principal factors affecting the joint property of ultrasonically welded thermoplastics. A weldability diagram was proposed based on the statistical results to give first guidelines for system optimization. In addition, amorphous polymers required less energy to be successfully welded than semicrystalline polymers. Semicircular energy directors were found to bond parts of highest strengths.