Joining of thermoplastics is an area of growing importance in the automotive, aerospace, electronics medical and other domestic appliance industries. While adhesive Bonding or mechanical fastening could be used to join thermoplastics, welding is very effective because of its speed and low cost. This study investgated the ultrasonic joining of thermoplastics. Four kinds of thermoplastics such as Acrylonitrile Butadiene Styrene, Polystyrene, Polyethylene and Polypropylene were used, ultilizing all possible joining combinations. In each combination of thermoplastics, the weldability of the joint was evaluated as a function of weld time, amplitude of vibration and pressure. It was generally found that joining of amorphous thermoplastics with semicrystalline thermopastics resulted in poor joints due to its different crystalline structure. Joining of the amorphous thermoplastics together and joining of the semicrystalline thermoplastics together produced the best joints owing to its same crystalline structure.