The article presents a statistical study which examines the parameters in ultrasonic welding of plastics (USW). The parameters being considered are the weld pressure, weld time, hold time, material type, joint type, and field type. A fractional factorial design was used to identify the individual effect of the welding variable and some of the interaction effects of these variables on the welding results. The study found that weld time can affect the welding results followed by the weld pressure and the interaction of the two parameters has also significant effect. On the other hand, hold time shows no significant effect on welding results.