This study was undertaken to elucidate bromine-antimon Synergism of the Polyester fabrics treated with a mixture of organobromine compounds/antimony trioxide. For this purpose fide aromatic bromine compounds with a relatively high bromine content such as DBDPO, OB-DPO, HBCD, PBT, and HBB were chosen. Polyester fabrics were treated with the aqueous dispersion solution of a mixture of bromine compounds/antimony trioxide together with binder. Bromine-antimony synergism was confirmed by plotting the LOI values against bromine/antimony weight ratio. Bromine-antimony synergism was inferred by comparing thermogravimetric and differential thermal analysis curves of a mixture of bromine compounds/antimony trioxide with those combined mathematically from thermogravimetric and differential thermal analysis curves of bromine compounds and antimony trioxide, respectively.