Phosphorus-nitrogen synergism in vinyl phosphonate oligomer (Fyrol 76) and N-methylol-acrylamide (NMA) treated cotton fabrics was studied through LOI and char length. And their thermal behavior under inert gas was also investigated through DSC, TGA and GC. When the total content of flame retarding element P and N was held constant, the maximum synergistic effect was observed at the P to N ratio 7:3-6:4. The DSC curves for Fyrol 76/NMA treated cotton fabrics under nitrogen atmosphere show an exothermic peak and two endothermic peaks. The second endothermic peak which is due to the decomposition of cellulose shifts 50-60$^{circ}$C lower than the untreated cotton fabric. In the GC and TGA analysis of Fyrol 76/NMA treated cotton fabrics, the amount of evolved CO2 and remained char increased as the amount of flame retardant material increased. These facts show empirically that the pyrolysis of flame retardant finished cotton fabrics follows the dehydration theory.