Abstract: Interfacial instability is an unsteady-state process in which the interface located between layers varies locally in a transient manner. This instability develops in the co-extrusion mold, and can be correlated with a critical interfacial shear stress for a particular polymer. In this work, a multilayer film co-extrusion experiment was conducted to measure the flow pressure drops on the mold wall along the flow to evaluate the flow shear stresses. Two pressure sensors were set sequentially on the mold wall along the flow direction for measuring the flow pressure drops. Pressure drops in the flow would be used to calculate flow shear stress. Variations of shear stress are then used to analyze interface slipping occur in the multilayer film. Two combinations of Polycarbonate (PC) / Polymethylmethacrylate (PMMA) and Polypropylene (PP) / Polycaprolactam (PA) were used as the experimental materials. Extrusion rate was experimentally increased to raise the pressure within the flow. According to the experiment results, when shear stress reaches a certain threshold, a large number of interfaces cause interfacial slip, which leads to a decline in shear stress. Increasing the number of interfaces increases interfacial slip. Moreover, PP/PA co-extruded film is more likely to cause slippage than PC/PMMA co-extruded film, because of its poor binding force between PP/PA interfaces.