Polycaprolactone (PCL) composites filled with nanometer calcium carbonate (nano-CaCO3) were prepared by means of a twin-screw extruder in this study. The nano-CaCO3 surface treated with stearate. The crystalline properties of the PCL/nano-CaCO3 composites were measured with a differential scanning calorimeter to identify the influence of the nanometer filler content on the crystalline properties. The results show that the crystallization onset temperature, crystallization temperature, and crystallization end temperature of the composites were obviously higher than those of the unfilled PCL resin, and the crystallization degree (c) of the composites increased with increasing particle weight fraction (phi f) when phi f was more than 1%. When phi f was 1%, c of the composite was less than that of the unfilled PCL resin. Moreover, the dispersion of the inclusions in the matrix was observed by means of scanning electron microscopy. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013