Synthesizing octylphosphate(OTP) from phosphorus pentoxide with n-octyl alcohol, octylphosphatethiourea(OTPU) was prepared from OTP with thiourea. After flame retardant finishing OTPU to acrylic fabrics, nylon taffeta and tetron taffeta, the physical properties, such as tear strength and flame retardancy were measured. As the results of the measurement, OTPU was found to be a good flame retardant having softness using to synthetic fibers.