The effect of ultrasonic oscillations on the weld line strength of amorphous polystyrene (PS), polymethyl methacrylate (PMMA), and PS/PMMA (20/80, 50/50, 80/20) blends at various temperatures was investigated. By facilitating the molecular diffusion across the weld line, the introduction of ultrasonic oscillations could evidently improve the weld line strength of PS, PMMA, and their blends. The different effects on the weld line strength of PS/ PMMA (20/80, 50/50, 80/20) blends were investigated. The ultrasonic oscillations could greatly increase the weld line strength of PS/PMMA (80/20) by similar to 70%, but was less efficient to PS/PMMA (50/50, 20/80) blends, due to the great difference of weld line morphologies of these blends. The dispersed phase of PS/PMMA (80/20) in the weld line was spherical while two different morphologies in the weld line of PS/PMMA (50/50) were observed. And the stripe-like morphology of PS perpendicular to the flow direction in the weld line of PS/PMMA (20/80) is responsible for the little effect of ultrasonic oscillations. The fractured surfaces of PS, PMMA, and PS/PMMA (80/20) with weld line became much rougher due to the introduction of ultrasonic oscillations. The morphology study of PS/PMMA (80/20) showed that the spherical dispersed phase of PS/PMMA at the skin turned smaller under ultrasonic oscillations. (c) 2006 Wiley Periodicals, Inc.