The main goal of this research is to investigate the effect of nano-CaCO3 coated with different content of stearic acid on fracture toughness of HDPE/CaCO3 nanocomposite. For this purpose, HDPE/10 vol % CaCO3 nanocomposites were made using a twin-screw mixer–single-screw extruder. Nanocomposite standard samples were prepared via injection molding method. Three-point bending tests and microscopic evaluations were performed. The results of fracture toughness test showed that the addition of uncoated nano-CaCO3 to HDPE causes to decrease fracture toughness, but it can be increased slightly as stearic acid content added. Also the results showed that the stress whitening zone width depends strongly on both CaCO3 and stearic acid content. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 3688–3694, 2007