A comparative study was performed of fly ash and nano-CaCO3 as fillers in polybutadiene rubber with 0, 4, 8 and 12% fly ash and nano-CaCO3. Uniform sheets were prepared of well-compounded rubber. Nano-CaCO3 was synthesized by in situ deposition. The CaCO3 nanoparticles as reinforcing agents improved the tensile strength more than 50%, than fly ash, and the toughness and hardness also increased significantly. Up to a 75%, reduction in flammability and a 100% improvement in the tear strength were observed with nano-CaCO3. (C) 2005 Wiley Periodicals, Inc.