For the development of technology for durable flame retardant finish of PET fabrics, an experimental study was carried out, The following conclusions were drawn: 1. In the synthesis of diphenyl methyl phosphate(DPMP) and diphenyl ethyl phosphate(DPEP) which served as flame retardant for PET, the process using nitrogen current to expel HCl gas in the reaction system is practicable without the use of solution such as pyridine or tertiary amine. 2. DPMP and DPEP showed good flame retardancy effects. 3. PET fabrics treated with DPMP and DPEP were excellently durable to repeated washings and wet cure gave softer touch than dry cure.