This article describes the thermal degradation behavior of polycarbonate-polydimethylsiloxane (PC-PDMS) block copolymer with dimethylsiloxane (DMS) block size 15 to 350 units, and the effects of the PDMS block size and the PDMS content on thermal degradation were studied. PC-PDMS block copolymer with DMS unit of 100 had the lowest value of maximum weight loss rate and the most residue containing silica in the other PC-PDMS block copolymers. The PDMS block size influenced PDMS dispersibility in PC and the moderate PDMS dispersion (similar to 50 nm) caused high flame retardancy for PC. The control of nanodispersion of PDMS caused the change of thermal degradation behavior and high flame retardancy in PC. (c) 2006 Wiley Periodicals, Inc.