Based on the novel USW method, the welds of Polylactide (PLA) - Polyformaldehyde (POM) and PLA - polymethyl methacrylate (PMMA) have been carried out. The interdiffusion processes of polymer chains have also been investigated according to the reptation theory and the hauling theory. The welding strengths are carried out by the universal tester and the rupture interfaces are observed using the scanning electron microscope. The best welding strength is 52MPa (90% compared with PLA of the PLA-POM system), which can reach the tensile strength of IPS. The molecules of POM are much easier to inter-diffuse than those of PLA and PMMA.