Compared with aramid fabrics, ultrahigh molecular weight polyethylene fiber unidirectional (UHMWPE UD) composites have higher ballistic limited velocity at the same areal density, making UHMWPE UD composites better candidate materials for the design of body armor against ballistic and stab impact. In this study, UHMWPE UD composites impregnated with thermoplastic films were prepared, and a novel methodology was proposed to evaluate the stab‐resistant performance of these composites using the concept of stab impact. Stab‐resistant performance is strongly affected by stab impact parameters, such as stab velocity or impact energy, layers of UD, and stab angle. Results show that the stab‐resistant performance of composites can be significantly improved by thermoplastic films, particularly polyethylene terephthalate and polypropylene films. The structure of UD composites functions importantly in enhancing stab resistance. Copyright © 2014 John Wiley & Sons, Ltd.