Abstract: Factors that affect the quality of ultrasonically welded joints were investigated from point of view of their melting states. Physical parameters such as the temperature, viscosity and thickness of melting layer of plastic material joints were measured and analyzed. Results show that the temperature increases, the viscosity decreases, the thickness of melting layer decreases, with the increasing of welding vibration amplitude and pressure. The microstructure of weld fusion zone was observed by using optical microscope. It is also found that there is strong orientation along transverse direction in the microstructure of the fusion zone, and the tendency of the microstructure orientation increases with the shearing rate. Testing results show that welded joints possess obviously anisotropic mechanics performance, strongly affected by the thickness of melting layer and the degree of orientation. Welding process should be rationally determined to achieve appropriate thickness of melting layer and degree of microstructure orientation for satisfactory performance of welded joints.