Cotton fabrics were treated with THPS-urea of various mole ratios of THPS to urea prepared by adding urea to a THPS-urea precondensate whose mole ratio of TRPS to urea was 1 to 0.67. The treatment condition imparting maximum flame retardancy to cotton fabric was investigated by LOT measurement, and the physical properties of treated fabrics such as tensile strength, tearing strength and stiffness were also investigated. The optimum treatment condition imparting maximum durable flame retardancy was found when the mole ratio of THPS to urea was 1 to 1.5, and the fabric treated under these conditions passed the DOC FF 3-71 flame test after 50 laundry cycles.