A nanosize CaCO3 filler was synthesized by an in situ deposition technique, and its size was confirmed by X-ray diffraction. CaCO3 was prepared in three different sizes (21, 15, and 9 nm). Styrene-butadiene rubber (SBR) was filled with 2-10 wt % nano-CaCO3 with 2% linseed oil as an extender. Nano-CaCO3-SBR rubber composites were compounded on a two-roll mill and molded on a compression-molding machine. Properties such as the specific gravity, swelling index, hardness, tensile strength, abrasion resistance, modulus at 300% elongation, flame retardancy, and elongation at break were measured. Because of the reduction in the nanosize of CaCO3, drastic improvements in the mechanical properties were found. The size of 9 nm showed the highest increase in the tensile strength (3.89 MPa) in comparison with commercial CaCO3 and the two other sizes of nano-CaCO3 up to an 8 wt % loading in SBR. The elongation at break also increased up to 824% for the 9-nm size in comparison with commercial CaCO3 and the two other sizes of nano-CaCO3. Also, these results were compared with nano-CaCO3-filled SBR without linseed oil as an extender. The modulus at 300% elongation, hardness, specific gravity, and flame-retarding properties increased with a reduction in the nanosize with linseed oil as an extender, which helped with the uniform dispersion of nano-CaCO3 in the rubber matrix. (c) 2005 Wiley Periodicals, Inc.