This report describes the flame retardancy and the thermal degradation behavior of polycarbonate-polydimethylsiloxane (PC-PDMS) block copolymer/silica nanocomposites. PC-PDMS block copolymer with dimethylsiloxane (DMS) block size 40 units increased the dispersibility of nanosized amorphous silica. Addition of the slight nanosized silica caused the increment of flame retardancy of PC-PDMS block copolymer, and the PC-PDMS block copolymer with 1.0 wt % PDMS had the highest limiting oxygen index value when the nanosized silica was added 0.5 wt %. The maximum rate temperature of the PC-PDMS block copolymer increased with the addition of silica and the maximum loss rate was the lowest when silica content is 0.5 wt %. The monodisperse nanosized silica had an effect that enhances the flame retardant mechanism of PDMS for PC. (c) 2006 Wiley Periodicals, Inc.