In the multilayer co-extrusion process, two or more materials are simultaneously brought together and layered in a single extrusion process. When co-extrusion two polymers with different rheological properties, the shear stresses occurred between two adjacent layers have important influences on the interfacial stability. This work aimed to investigate the effects of the number of layers on the flow shear stress. We conducted a multilayer film co-extrusion experiment to measure the melt pressures on the die wall along the flow to evaluate the flow shear stresses. Moreover, a high speed camera was employed to record the interfacial behavior of the multilayer flow. Two combinations of PC/PMMA and PP/PA were used as the experimental materials. The experimental results show that the flow shear stress in a co-extrusion process decreases with the increase of the number of layers. This implies that slip may happen in the interface of two adjacent flow layers. Keywords: co-extrusion, multilayer film, slip