The effect of interfacial morphology controlled by injection-molding conditions on the welding strength of injection-molded polyamide was investigated in this article. The experimental results showed that the first injection-molding conditions had distinct influence on the welding strength at the low secondary injection-molding temperature T-2 ( <= 265 degrees C), but the influence vanishes at T-2 >= 285 C. On the other hand, no matter what the first injection-molding conditions are, the welding strength increases with increasing T-2, and when T-2 >= 285 degrees C, the highest welding strength reaches 45 MPa, due to the formation of trans-crystals at the interface. Morphology studies showed that trans-crystals grow along the perpendicular direction of the interface, and their nuclei are formed at the surface of the first injection-molded specimens. For the specimens with high-welding strength, the welding strength relies on the skin layer of the first injection-molded specimens in which the fracture induced by shear stress happens. POLYM. ENG. SCI, 47:2164-2171, 2007. (C) 2007 Society of Plastics Engineers.