Polyethylene (PE) is one of the most produced synthetic resins in the world. Functionalized multiwalled carbon nanotube (MWCNT) is a potential nanoscale filler to realize the next generation strong and multifunctional PE composites. We demonstrate that MWCNTs grafted with short n-alkyl groups are effective nanofillers for mechanical reinforcement of PE composites. At 1 wt% filler loading, the yield stress and Young's modulus improve significantly up to 54 and 63% compared with neat PE, respectively. Among ductile properties, tensile strength increases up to 30%; ultimate strain is preserved; and toughness increases up to 33%. More important, we show that short n-alkyl groups can be grafted on MWCNTs much easier than long chain polymers. Further, we find that alkyl groups of C14-C18 chains have the optimum length for reinforcement. The optimum density of grafted alkyl groups is around 0.390-0.423 mu mol/mg when the C14 alkyl groups are used. Overall, the results manifest that MWCNTs grafted with n-alkyl groups with suitable length and density are efficient nanoscale fillers for high-performance PE composites. POLYM. ENG. SCI., 54:336-344, 2014. (c) 2013 Society of Plastics Engineers