Poly(?-caprolactone) (PCL)/titanium dioxide (TiO2) nanocomposites were prepared by in situ polymerization of ?-caprolactone in the presence of modified-TiO2 nanoparticles as initiators. The molecular weight of PCL matrix was dependent on the amount of the TiO2 fillers. The incorporation of TiO2 did not significantly affect the crystalline structure of PCL. Moreover, a tendency of the nanoparticles to form aggregates was observed, especially at higher fillers contents. The analysis of the crystallization process showed that the addition of TiO2 nanoparticles accelerated the crystallization rate of PCL, and the crystallization rates increased by increasing the filler content. The crystallization activation energy dependence on the filler content observed here is probably the consequence of the two competing factors. The tendency of activation energy obtained by nonisothermal crystallization is similar to that of isothermal crystallization. POLYM. ENG. SCI., 2012. (C) 2011 Society of Plastics Engineers