The factors influencing the mechanical properties of styrene-butadiene-styrene block copolymer (SBS) composites filled with liquid polybutadiene (LB)-surface-treated calcium carbonate (CaCO3) were investigated with respect to the molecular structure of the LB, the amount of the LB adsorbed on the CaCO3 surface, the heat treatment conditions, and the surface treatment method. The mechanical properties, such as the modulus, tensile strength at break, tear strength, storage modulus, and tension set, of the SBS composites were improved remarkably through the filling of CaCO3 surface-treated with a carboxylated LB with a high content of 1,2-double bonds. The heat treatment of LB-CaCO3 in air was also effective in enhancing such properties. When SBS, CaCO3, and LB were directly blended (with the integral blend method), secondary aggregation of CaCO3 place, and the mechanical properties of the composite were significantly lower. In the integral blend method, LB functioned as a plasticizer. (C) 2009 Wiley Periodicals, Inc. J Appl Polyrn Sci 113:3661-3670, 2009