Dichlorotribromophenyl phosphate (DCTBPP) was synthesized via the reaction of tribromophenol and phosphorous oxychloride and characterized by elemental analysis, IR, 1 H-NMR, thermogravimetric analysis, and differential scanning calorimetry. To impart durable flame retardancy the poly(ethylene terephthalate) (PET) fabric was treated with DCTBPP via pad-dry-thermosol fixation and the PET/cotton (50/50) blend fabric was treated with both DCTBPP and tetrakis(hydroxymethyl) phosphonium chloride (THPC)/urea precondensate via a two-bath sequential treatment. The treated PET fabric's increased limiting oxygen index value was proportional to the increasing DCTBPP application level and showed self-extinguishing properties at 8.1% add-on, even after 50 washes. The blend fabric treated with 15% DCTBPP and 30% THPC/urea precondensate became self-extinguishable and durable to 50 washes, and the treated fabric retained over 85% of its breaking strength without excessive stiffness.