A novel halogen-free phosphorus-nitrogen-silicon flame retardant monomer with reactive siloxy groups, N-(diphenylphosphino)-1,1-diphenyl-N-(3-(triethoxysilyl) propyl) phosphinamine (DPTA) has been synthesized and was applied to the fire-resistant finishing of cotton fabrics. The molecular structure of DPTA has been well characterized by elemental analysis, FTIR, H-1 NMR, and P-31 NMR spectroscopies. The chemically-grafted cotton fabrics, which were treated with 25 wt% DPTA, were obtained and confirmed by attenuated total reflectance Fourier infrared spectroscopy (ATR-FTIR). The flame retardancy and thermal property of the treated samples were investigated by limited oxygen index (LOI), vertical flammability test (VFT), thermogravimetric analysis (TGA) and microscale combustion calorimeter (MCC). It is noted that in vertical flammability test, the treated samples extinguished immediately upon removing the ignition source, whereas the untreated one was completely burned out. Furthermore, TGA and MCC tests revealed that the treated samples produced a high char formation and a low heated release during combustion. The surface morphology of the untreated and treated samples and the char residues after LOI tests were observed by scanning electron microscopy (SEM). Therefore, all the results showed that the treated cotton fabrics with 25 wt% DPTA apparently improved the fire-resistant and thermal performances. (c) 2016 The Chemical Industry and Engineering Society of China, and Chemical Industry Press. All rights reserved.