This is a comparison study of the effects of various colloids on polydimethylsiloxane (PDMS) nanocomposites. Mica of layered structure and silica of nanospherical shape were both used with different content. Silane coupling agent was used for surface modification of mica to improve its dispersion. Wide X-ray diffraction patterns showed no intercalation during the incorporation of mica into PDMS matrix. Rheological characterization of the PDMS composites before curing showed that viscosities of PDMS-mica composites were much lower than that of PDMS composites with silica, particularly at low shear rates. Scanning electron microscope (SEM) suggested the existence of mica lamellar structure with high aspect ratio within the PDMS matrix. After vulcanization at room temperature, PDMS-mica composites could obtain mechanical properties close to that of PDMS composites with silica at same filler content. The cooperative effects of mica with silica showed that same mechanical properties of PDMS composites could be obtained at much lower viscosity, cost, and processing insecurity. (C) 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 127: 3017-3025, 2013