The morphological evolution of ethylene–vinyl alcohol copolymer (EVOH) and its effect on the gas-barrier properties of high-density polyethylene (HDPE) were investigated. HDPE/EVOH blends were prepared through a multistage stretching extrusion, which combined an assembly of force-assembling elements (FAEs) with an extruder. Scanning electron microscopy confirmed that with an increasing number of FAEs, the biaxial-stretching field existing in each FAE transformed the dispersed EVOH phase into well-defined platelets along the flowing plane. Dynamic rheological results further revealed that the formation of the platelets enlarged the interfaces between the dispersed barrier phase and the matrix; this not only led to the decline of the complex viscosity but also created more tortuous paths for the diffusion of gas molecules. Compared with that of the non-FAE specimen blended with 25 wt % EVOH, the oxygen permeability coefficient decreased more than one order of magnitude when one FAE was applied. The structural model for permeability indicated that the enhanced barrier resulted from the increased tortuosity of the diffusion pathway, which was provided by the aligned high-aspect-ratio platelets. Compared with the previous biaxial-stretching method, multistage stretching extrusion provided a simple and economical way to generate a laminar structure of the dispersed phase in the matrix phase without the application of an external stretching force. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40221.