In this study, multiwalled carbon nanotubes (MWCNTs) were dispersed into a poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) matrix, in which PHBV was either covalently attached to the nanotubes through an esterification reaction between the carboxylic groups of functionalized MWCNTs and the hydroxyl groups of PHBV with toluene diisocyanate as a coupling agent or physically mixed to result in only noncovalent interactions. The structure, crystallization behavior, and thermal properties of the resulting nanocomposites were studied. We found that the crystallization of PHBV grafted onto the MWCNTs (PHBV-g-MWCNTs) was markedly hindered and exhibited an exothermic peak caused by cold crystallization, whereas the nonisothermal crystallization of PHBV was enhanced because a heterogeneous nucleation effect appeared in the PHBV/MWCNTs. Moreover, the maximum decomposition temperature of the PHBV-g-MWCNTs was improved by about 14.4 degrees C compared with that of the PHBV/MWCNTs and by about 23.7 degrees C compared with that of the original PHBV. Furthermore, the PHBV-g-MWCNTs exhibited the wider melt-processing window than the PHBV/MWCNTs and original PHBV. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4299-4307, 2013