The effect of the addition of silica particles on spherulite growth rates of isotactic polypropylene has been investigated using an optical microscope. The spherulite growth rate remained constant with time and the growth rates decreased with increasing the silica content. The addition of silica particles generally causes a reduction of the average distance between adjacent silica particles and the linear growth rates become zero when the inter-particle distance reached to the end-to-end distance of the matrix chains. The reduction in the linear growth rate can be explained by assuming the lamellar clustering process that the precursor or solidified units with the single chain space, which spontaneously are built up from the melt, act as the crystallizing units and the diffusive mobility of the units was restricted by spatial confinement between adjacent silica particles. (c) 2006 Elsevier Ltd. All rights reserved.