This report describes the flame retardancy of a polycarbonate (PC)-polydimethylsiloxane (PDMS) block copolymer with a dimethylsiloxane (DMS) block size of 15-350 units, and the effects of the block size and amount of DMS on the flame retardancy are studied. PC-PDMS block copolymers with DMS units of 40-130 had high limiting oxygen index values with 1.0 wt % PDMS. The PDMS block size influenced the PDMS dispersibility in PC, and a moderate PDMS dispersion (ca. 50 nm) caused high flame retardancy for PC. These PC-PDMS block copolymers could form a lot of fine bubbles in the role of good thermal insulators through the reaction of PC and PDMS in combustion. Furthermore, the silica particles from PDMS remained mostly on the surface of the char, so the amount of char with high oxidation resistance increased. (c) 2006 Wiley Periodicals, Inc.