Maleated high-density polyethylene (mPE) consisting of 95 wt% high-density polyethylene and 5 wt% polyethylene-graft-maleic anhydride was filled with 2-4 wt% silicon carbide nanoparticles (SiC(p)) and toughened with 10-30 wt% poly(styrene-ethylene-butylene-styrene) (SEBS). Such mPE nanocomposites were compounded in a twin-screw extruder followed by injection molding. The effects of SEBS elastomer and SiC(p) additions on the structure, thermal and mechanical behavior, and fracture toughness of mPE composites were studied using X-ray diffraction (XRD), polarizing optical microscopy (POM), differential scanning calorimetry (DSC), heat deflection temperature (HDT), tensile test, Izod impact, and essential work of fracture (EWF) techniques. XRD and POM results showed that SEBS and SiC(p) additions reduce the crystallite thickness and spherulite size of mPE. DSC and HDT measurements revealed that SiC(p) addition promotes nucleation of HDPE crystals and markedly increases the range of elevated temperature over which mPE hybrids maintain adequate stiffness. Tensile test results showed that SEBS reduces the Young's modulus and yield strength of mPE, whereas SiC(p) stiffens and reinforces the mPE/SEBS blend. The Izod impact test was conducted at liquid nitrogen temperature. The impact strength of mPE/SEBS/SiC(p) hybrids improved dramatically with increasing SEBS content. EWF results obtained at room temperature showed that the specific EWF of mPE/SEBS blends and mPE/SEBS/SiC(p) hybrids also increases with increasing SEBS content. (C) 2011 Wiley Periodicals, Inc. Adv Polym Techn 30: 322-333, 2011; View this article online at wileyonlinelibrary.com. DOI 10.1002/adv.20228