||Co-injection molding has been introduced into industrial application for several decades. However, due to the formation of the interface between skin and core materials is very difficult to be observed, and controlled, a good quality of co-injection product can not be obtained effectively. The reason is that the formation of that interface in co-injection molding is very sensitive to various factors. In this study, the formation of the interfacial morphology and its physical mechanism in co-injection molding have been studied based on the ASTM D638 TYPE V system by using both numerical simulation and experimental observation. Results showed that the critical skin/core material ratio to generate the skin break-through is identified. The reason to cause the break-through is due to the flow front of core material catches up with the melt front of skin, and the skin is stop at a fixed distance. This mechanism is similar with that of literature. However, when the higher core material ratio is selected, the mechanism of the interfacial morphology is different. Specifically, after core melt front catches the skin melt front, the broken skin material can move forward with the inner core material to generate special core-skin-core structure. It could be due to different forces balance inside the skin and core melts, but needs to do more study in the future.