Poly(trimethylene terephthalate)-poly(ethylene oxide terephthalate) block copolymer (PTG)/multiwalled carbon nanotubes (MWCNTs) composites were prepared via in situ polymerization. To improve the dispersion of MWCNTs in the PTG matrix, the poly(ethylene glycol)-grafted multiwalled carbon nanotubes (MWCNT-PEG) were produced by the graft to method. The transmission electron microscopy observation demonstrated that a homogeneous dispersion of MWCNT-PEG was obtained. As a consequence, the percolation threshold for the rheology was around 0.5 wt% and the conductivity was approximate to 1 wt%, respectively. Differential scanning calorimetry and polarized optical microscopy results confirmed that MWCNT-PEG can act as an effective heterogeneous nucleating agent. Interestingly, the effects of MWCNT-PEG on crystallization and melting of the poly(ethylene oxide terephthalate) blocks were more pronounced than on those of the PTT blocks. POLYM. ENG. SCI., 2013. (c) 2012 Society of Plastics Engineers