Abstract “Encapsulation phenomena”, which always caused the higher viscosity fluid sandwiched by the lower viscosity fluid, has been observed several decades ago, which was regarded as an important factor influencing stratified flow in co-extrusion. In this work, an innovative method for forming multilayer structure was illuminated, which attributed to the interesting “viscous encapsulation” phenomenon. By controlling of molecular factors and processing conditions, a stable stratified flow was obtained as a result of phase-segregation, trilayer and five-layer structure were formed in slit die extrudate interestingly. The influencing factors, such as molecular weight distribution, viscosity ratio, shear rate, length/diameter ratio (L/D) of capillary die were discussed in this paper. It was found that the narrow MWD HDPE/PP binary system could have a high A p (area percentage) leading to the formation of “core-shell” structure while wide MWD HDPE/PP binary system may lead to a special sandwich structure in capillary die extrudate within low shear rate region. Our work also illustrated two dominate parameters of preparing multilayer structure: viscosity ratio and shear rate. The high viscosity ratio ensured rapid radial movement during flow and a suitable shear rate provided enough time for molecular chains to separate and developed into a clear core-shell structure. A high value of A p could be achieved from shear rate of 45 s − 1 –175 s − 1 , which could be a reasonable processing zone. A p . It was found that the different resultant force applied on HDPE and PP granules during stratified flow has resulted in the formation of “core-shell” structure. The simulation results also showed a strong tendency for PP to flow along the wall under pressure flow while HDPE migrated to the opposite direction. The extrudates from capillary die and extrusion slit die were observed and compared with POM, The value of A p from extrusion slit die was 91% which was greater than it in capillary die due to pre-flow along radial direction in screw rotation.