Blends of polystyrene and polyethylene (PS/PE), including blends in which a styrene/ethyrene-butylene/styrene (SEBS) terpolymer was employed as a compatibilizer, were studied. Their rheology showed that the effect of the addition of SEES to PS/PE blends was strongly affected by the blend composition and the shear rates involved in the blending and post-forming processes. The addition of PE to PS led to a reduction of fracture toughness compared with that of PS. This effect was attributed to the fine minor phase morphology of the blends obtained after extrusion blending and injection molding. The fatigue crack propagation (FCP) results showed that the fatigue crack growth rates were significantly reduced at low and moderate range of stress intensity factor (Delta K) by the presence of PE. Performance was enhanced when SEES was present. The results also showed that both the fracture toughness and the FCP behavior of the blends were strongly dependent on the loading direction, the minor phase morphology, the composition of the blend, and, to a lesser degree, the presence of a compatibilizer. This study demonstrates that the fracture toughness and the FCP performance of such polymer blends can vary inversely.