This paper discusses the weldability of similar and dissimilar thermoplastics with the use of ultrasonic welding and compares the experimental results with those obtained previously by using the heated tool method. Theoretical discussions presented on the effects of diffusion, solubility parameter, surface tension, contact pressure, thermal expansion, thermal dissipation, and vibration transmissibility are used for comparing weldability. Weld strength-to-bulk strength ratios and the ultimate extension values for the welded specimens are compared fur assessing the weld efficiency, and therefore the quality of autohesion (self-adhesion), and adhesion. In experiments with ultrasonic welding, the horn down speed is varied to assess, indirectly, the effect of contact force during welding. A sandwich weld structure and the effects of specimen length on the weld strength are also studied for ultrasonic applications to assess these geometrical effects. It is shown that a threshold length exists for weld specimens over which the weld strength drops off considerably. Furthermore, it seems that a sandwiched structure can be used to obtain further information on energy transmission and transformation for ultrasonic welding.