A poly(ethylene-cb-propylene) comb block copolymer (P(E-cb-P)), prepared by copolymerization of vinyl-terminated atactic polypropylene and ethylene, was used to compatibilize immiscible blends of high-density polyethylene (HDPE) and isotactic polypropylene (iPP). Addition of 5 wt % P(E-cb-P) resulted in 5-fold microdomain size reductions and the concomitant increase in the elastic modulus, as typically observed in immiscible blends compatibilized with linear block copolymers. We report an PE Om unexpected phenomenon, namely, the development of extensional flow hardening by the addition of P(E-cb-P) to the HDPE/iPP blends. This unprecedented effect is stronger in blends with cocontinuous morphology (50/50 HDPE/iPP) than in blends with matrix-droplet morphology (75/25 or 25/75 HDPE/iPP). We postulate that the melt strength enhancement and extensional strain hardening observed in the compatibilized blends may arise from the interfacial stiffening as a result of the interfacial stitching by the P(E-cb-P) comb block copolymer. This interfacial stitched network acts as an elastic membrane that resists interfacial deformations. Entanglements of the PP comb arms with iPP generates interfacial stitches which, in turn, could lead to the stretching of the PE backbone of the P(E-cb-P) comb block at large interfacial deformations and, hence, extensional flow hardening.