This study was carried out to investigate the flame-retardant mechanism of the polyester fabrics treated with a mixture of organobromine compounds/antimony trioxide. For this purpose polyester fabrics were treated by pad-dry-cure method with the aqueous dispersion solutions of DBDPO/antimony trioxide or HBCD/antimony trioxide using a binder. The flame-retardant mechanism was studied by the limiting nitrous oxide index (LNOI) and char analysis. The LNOI values hardly changed with the increase of bromine contents. The results of char analysis showed that most of the bromine presented in treated fabrics had volatilized during the pyrolysis for 3 minutes at 50$0^{circ}C$. The gas phase mechanism was found to operate in polyester fabrics treated with combination of Br/antimony trioxide. This was also confirmed by comparison with the results of LNOI and char analysis of DAP treated polyester fabrics of the condensed phase mechanism.