The present research is aimed at studying the structural effect of two phosphoramidate derivatives diethyl 3-hydroxypropylphosphoramidate EHP and dimethyl 3-hydroxypropylphosphoramidate MHP as flame retardants for cotton fabric. EHP and MHP were obtained in very high yield and purity by one step procedures. Cotton twill fabrics treated with the two compounds at different add-ons (5-20 wt %) were characterized. Vertical flammability, limiting oxygen index, thermogravimetric, and microscale combustion calorimeter analyses were performed, and all resulted in better flame retardancy and thermal behavior for MHP compared to EHP. A study of the functional groups which appeared on the treated fabrics by attenuated total reflection infrared spectroscopy revealed different binding mechanisms between each compound and cotton cellulose. Analysis of the released gas products by thermogravimetric analysis-Fourier transform infrared spectroscopy showed some distinctive details in the degradation of the treated fabrics during the burning process.