A new flame retardant diphenyl butylamidophosphate (DPBAP) for PET fabric was synthesized and its flame retardancy was examined. The results have shown that PET fabrics treated by DPBAP(with DPBAP and on 4-10 %) show excellent flame retardancy. Since the DPBAP treated PET fabric show essentially no change in the drape stiffness and the tensile strength, it is believed that DPBAP is chemically stable in PET fabric. In addition to this, the washing fastness of DPBAP on PET fabric tested by the 5 times of water washing method also appeared to be excellent. Judging from the fact that DPBAP was synthesized from relatively cheap material as well as the above cited DPBAP properties as a good flame retardant, the potential of DPBAP to be developed as a commercial flame retardant for PET fabric seems to be high.