Technical Abstract: Urea is well known to have a synergistic action with phosphorus-based flame retardants such as diammonium phosphate (DAP) in enhancing the flame retardant performance of cellulosic materials, but its effect on their thermal decomposition kinetics has not been thoroughly studied. In this study, the activation energy (Ea) of the thermal decomposition of greige cotton nonwoven fabrics, which were treated with different concentrations of urea at fixed contents of DAP, was measured using Kissinger, Friedman, and Flynn-Wall-Ozawa methods. With a good agreement between these methods, the addition of urea at low concentrations increased the Ea above the Ea obtained from DAP treatment alone, but the excess of urea significantly lowered the Ea. These effects of urea were associated with a continuous increase of its synergistic function observed as increasing the addition of urea. Up to a certain concentration, which depends on the amount of phosphorus, urea improves the thermal stability of cellulose by assisting the DAP reactions involving pyrophosphate formation and phosphorylation. The other effect, which was predominant at high concentrations of urea, is to facilitate the decomposition of cellulose at temperatures lowered by the dehydration of DAP. These explanations were supported by the analyses on the thermal decomposition process of greige cotton fabrics and charred fabrics using thermogravimetry, 31P MAS NMR, ATR-FTIR, and EDAX.