The effect of silica particles (silica), which are petroleum-free products, on the isothermal and nonisothermal crystallizations of cis-1,4-poly(butadiene) rubber (BR) is investigated. DSC measurements reveal that silica accelerates the crystallization of BR. As a result, BR-containing silica shows crystallization even after crosslinking reaction. Because of the enhanced crystallization, the tensile modulus is much higher than that without silica in the range between the glass transition temperature and the melting point. (c) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013