There is tremendous interest in using low loadings of multiwalled carbon nanotubes (MWNTs) to enhance the multifunctional properties of polymers, with functionalization often pursued to increase the dispersion and effective reinforcement of MWNTs within the polymer. In our interest to understand the effect of MWNT functionalization on Poly (butylene terephthalate) (PBT) crystallization kinetics, morphology and mechanical properties, nanocomposites were fabricated with both as-received and carboxyl group (-COOH) functionalized MWNTs. Initial results indicate as-received and functionalized nanotubes alter the crystallization temperature and crystal size for quiescent samples. In addition, isothermal crystallization studies using an Advanced Rheometric Expansion System (ARES) show that the addition of MWNTs increases the rate of PBT crystallization. However, functionalization was found to decrease the rate of nanocomposite crystallization as compared to nanocomposites samples prepared using pristine MWNTs, suggesting that nanotube functionalization weakens the nucleation effect observed in the nanocomposite samples. These results suggest that semicrystalline polymer nanocomposite crystallization kinetics and morphology can be significantly influenced by nanoparticle functionalization and chemistry. Further study of how these changes impact the rheological and multifunctional properties of semicrystalline nanocomposite systems are ongoing.