This study was carried out to investigate the flame-retardant mechanism of polyester fabrics treated with a mixture of bromine compounds/antimony trioxide. For this purpose polyester fabrics were treated by pad-dry-cure method with the aqueous dispersion solutions of decabromodiphenyl oxide/antimony trioxide or hexabromocyclododecane/antimony trioxide using a binder. The flame-retardant mechanism was studied by thermogravimetric analysis and confirmed by the residue number method. The flame retardancy was found to be operative in the gas phase mechanism. This was also confirmed by comparison with those of diammonium phosphate treated polyester fabrics of the condensed phase mechanism. Thermogravimetric analysis curves were considered to be one of effective tools to differentiate the flame-reame-retardant mechanism.