Nonisothermal crystallization kinetics of poly(butylene terephthalate) (PBT)/nano calcium carbonate (CaCO3) composites were investigated by differential scanning calorimetry and polarized optical microscopy. PBT/nano CaCO3 composites containing different wt.% of nano CaCO3 were prepared by melt blending. Analysis of the melt crystallization data by various macrokinetic models like modified Avarami and Liu and Mo models revealed the dependence of crystallization rate on nano CaCO3 content. Nano CaCO3 addition upto 5 wt.% greatly accelerated the crystallization rate. However, higher loadings of the filler retarded the crystallization rate. These results were further supported by the effective activation energy calculations by isoconversional method of Friedman. Polarized optical microscopy showed a decrease in spherulites size with increase in filler content. The study showed that small amount of nano CaCO3 acts as additional nucleating agent and is effective in accelerating the crystallization rate whereas higher loadings increase the nucleation density but restrict the mobility of polymer chains and retard the growth process. (C) 2015 Elsevier B.V. All rights reserved.