Experimental and theoretical studies were carried out to investigate the mechanism of the interface deformation in the stratified two‐phase flow of viscoelastic fluids through a rectangular duct. For the experimental study, a transparent test channel was constructed to permit visual observations, from the direction perpendicular to flow, on the interface formed when two incompatible molten polymers flow between two parallel planes. Extrudate samples were also collected, and the interface shape of the extrudate cross section was examined. For the theoretical study, using the Coleman‐Noll second‐order fluid as the constitutive equation, an approximate solution of the equations of motion was obtained numerically. The movement of the interface from the initially flat position was determined with the use of the normal stress continuity condition across the interface. The computed results show that the more viscous component tends to push into the less viscous component, consistent with experimental observation. It has been shown that the viscosity difference between the two components predominates over the elasticity ratio in determining the interface shape in stratified two‐phase flow.