Nonisothermal crystallization kinetics of virgin poly(butylene terephthalate) (PBT), PBT/nanoclay (NC) and PBT/carbon nanofiber (CNF) composites were investigated. The experimental data on crystallinity versus temperature were collected using a differential scanning calorimeter (DSC) at four different cooling rates (2.5,5,10 and 20°C/min). The data were analyzed using nonlinear multi-variable regression method and fitted to Avrami, modified Avrami, and Tobin models. The addition of carbon nanofiber and nanoclay in general improved the crystallization rate and lowered the onset and peak crystallization temperatures of the filled systems. These effects were dependent on the amount of reinforcement used and on the cooling rate applied, with more pronounced changes noticed at higher cooling rates. Previous results showed enhanced crystallization of polypropylene (PP) systems containing nanoreinforcements. The Avrami model proved to be the best fitting model among those investigated. The activation energies describing the overall nonisothermal crystallization process was also calculated using two different models: Kissinger and Takhor. The values obtained from these models were found to be almost identical. The addition of heterogeneous nuclei (carbon nanofibers or nanoclay platelets) to the PBT melt was found to lower the activation energy of virgin PBT. This result was related to an increased crystallization rate of filled PBT.