In recent years coextrusion process had gained wide recognition as an approach to achieving high quality or quantity and low costs by multilayered system as well as an polymer alloy or blend. We performed a strict 3-D numerical simulation with viscoelastic model on the encapsulation phenomena which is one of the problems in the coextrusion process. The effects of material properties such as viscosity ratio, extensional viscosity and second normal stress difference and an effect of flow rate ratios on the encapsulation phenomena were examined. Numerical results showed that encapsulation phenomena were affected by not only viscous properties but also elastic properties or non-linear properties. It was found that the difference of second normal stress differences working on the interface between fluid I and fluid II (we defined it as DN2) had a correlation with the interface shape.