An increase in the health and environment legislation awareness pushed the textile manufacturers to develop their strategies to produce eco-friendly flame-retardant textiles with competitive cost. Chitosan is added during the phosphorylation process as a nitrogen source that has synergistic effect with phosphorus. Increasing the chitosan concentration from 0% to 2% enhances the flame retardancy of the treated cotton fabric against successive washing. Further increase in amount of chitosan, above 2%, has a limited effect on the thermal degradation of the treated cotton. Increasing diammonium hydrogen phosphate (DAHP) concentration from 0% to 10% is responsible for making the flame-retardant finish durable against successive washing. Thermal degradation analysis of treated fabrics at different DAHP concentrations shows a decrease in the maximum degradation rate point and thermal degradation onset point. The amount of residue at 500°C increases till reaches 32% with the increase in the DAHP concentration. The effect of curing temperature and time has been discussed in detail. The performance properties of the treated cotton fabrics show better physical and chemical properties than that in the absence of chitosan.