Flame retardant polycarbonate (FRPC) with 0.2 wt % additives of potassium perfluorbutane sulfonate and polydimethylsiloxane was found to achieve V-0 rate at 1.6 mm thickness without significantly affecting the mechanical properties of PC. Condensed aromatic bonds with a small amount of Si-O bonds were found in the UL-94 burning residues tested by FTIR spectra. A compact char layer with cavities inside was formed on the surface of the sample during the rapid decomposition of FRPC, and the concentration of Si was found to be much higher inside the surface of the char layer than that outside the surface analyzed by SEM-EDX, which was related to the synergistic effects of the two flame retardants of PPFBS and PDMS. A schematic diagram was designed to describe the mechanism of the FRPC. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 119: 2730-2736, 2011