The mechanical properties of poly(1-butene) reinforced by pristine multiwalled carbon nanotubes (MWNTs) and polypropylene-grafted MWNTs (PP-g-MWNTs) were evaluated. The incorporation of pristine MWNTs to PB led to an improvement in stiffness, but not in strength, ductility, and toughness. In comparison, PP-g-MWNTs were able to improve the stiffness, strength, and toughness of PB significantly, without compromising the ductility. The mechanical properties of PB improved with increasing amount of PP-g-MWNTs up to an effective MWNT content of 1.5 wt%. Further increase in the effective MWNT content led to a downturn in mechanical properties due to the existence of MWNTs bundles as observed by microscopy. (C) 2009 Wiley Periodicals, Inc. J Appl Polyrn Sci 113: 1165-1172, 2009