For the purpose of clarifying what mechanism is operative in the flame-retardant polyester fabrics treated with bromine compounds/antimony trioxide the calorimetry method was used. Polyester fabrics were treated by pad-dry-cure method with the aqueous dispersion solution of various concentrations or mole ratios of Br to Sb of DBDPO/antimony trioxide or HBCD/antimony trioxide using a binder. AH2 of treated fabrics was measured by the oxygen bomb calorimeter through thermodynamic consideration of combustion process and the effects of the above-mentioned variables on LOI and AH2 were examined. The flame retardncy was found to be operative in the gas phase mechanism. This was also confirmed by comparison with those of DAP treated polyester fabrics of the condensed phase mechanism.