In present study, DMDHEU (dimethylol dihydroxyethyleneurea), which has been used for wrinkle-resistant finishing in textile, was used in FR finishing of CO/PET blends as a crosslinker with novel FR chemical (Fire-off). Flame retardancy performance of CO/PET fabrics treated with Fire-off and Fire-off/DMDHEU were investigated and compared. In addition, wrinkle resistant test was performed for untreated and Fire-off/DMDHEU treated fabrics to investigate the effect of the treatment on wrinkle recovery performance of fabrics. Results showed that Fire-off/DMDHEU system is a more effective nitrogen provider than Fire-off alone to enhance the flame retarding performance of the treated CO/PET fabrics through phosphorus-nitrogen synergism, therefore the presence of DMDHEU in the flame retardant finishing system increases the flame resistance of the treated CO/PET fabric. In addition, DMDHEU does not significantly change the wrinkle recovery of fabrics when used with Fire-off.