Carbon nanofibers (CNFs) were covalently functionalized with biodegradable poly (epsilon-caprolactone) (PCL) by in situ ring-opening polymerization (ROP) of epsilon-caprolactone in the presence of stannous octoate. Surface oxidation treatment of the pristine CNFs afforded carboxylic CNFs (CNF-COOH). Reaction of CNF-COOH with excess thionyl chloride (SOCl2) and glycol produced hydroxyl-functionalized CNFs (CNF-OH). Using CNF-OH as macroinitiator, PCL was covalently grafted from the surfaces of CNFs by ROP, in either the presence or absence of sacrificial initiator, butanol. The grafted PCL content was achieved as high as 64.2 wt %, and can be controlled to some extent by adjusting the feed ratio of monomer to CNF-OH. The resulting products were characterized by FTIR, NMR, Raman spectroscopy, TGA, DSC, SEM, TEM, HRTEM, and XRD. Core-shell nanostructures were observed under HRTEM for the PCL-functionalized CNFs because of the thorough grafting. The PCL-grafted CNFs showed different melting and crystallization behaviors from the mechanical mixture of PCL and CNF-OH. This approach to PCL-functionalized CNFs opens an avenue for the synthesis, modification, and application of CNF-based nanomaterials and biomaterials. (c) 2007 Wiley Periodicals, Inc.