Polyester fabrics were treated with decabromodiphenyl oxide and antimony trioxide by use of binder. The synergistic interactions of bromine and antimony were studied. The LOI values increased with the content of DBDPO or antimony trioxide in fabrics, but flame retardancy was not satisfactory when they were used independently. On the contrary, when a mixture of DBDPO and antimony trioxide was used, the synergistic effects were remarkable and the maximum LOI value was obtained at 715 Br/sb ratio in treated fabrics. When the Br contents in the fabrics of 713 Br/sb ratio were more than 8.5% , flame retardancy was also sustained after 5 laundering cycles. Flame retardancy of these fabrics exhibited the gas-phase mechanism.