High density polyethylene (HDPE)/organoclay nanocomposites toughened with maleated styrene-ethylene-butylene-styrene elastomer (SEBS-g-MA) were prepared by melt compounding. The structure and mechanical properties of such nanocomposites were investigated by X-ray diffraction (XRD), Fourier transform infrared spectrometry (FTIR), tensile and impact techiniques. XRD measurements showed that meleic anhydride group of SEBS elastomer was beneficial in forming an intercalated or partially exfoliated structure. The essential work of fracture (EWF) approach under tensile condition was used to characterize the fracture toughness of nanocomposites. The results indicated that the addition of SEBS-g-MA to the HDPE/organoclay nanocomposites greatly enhanced their fracture toughness.