Polyphosphoramido sulphides were synthesized from aryl phosphoramidic dichlorides and aryl thiophosphoramidic dichlorides with sodium polysulphide in aqueous acetone medium at room temperature. These polymers were characterized by elemental analysis and IR spectroscopy. Their thermal and flammability characteristics were evaluated by thermogravimetry and limiting oxygen index data respectively. These polymers were used as flame retardant for hydroxyl terminated polybutadiene rubber. The flammability and mechanical properties were studied for the cured HTPB rubbers. Higher sulphur content in the structure enhances the LOI and thus the flame retardancy. Polynaphthiophosphoramido sulphide emerges as a promising flame retardant amongsta four such polymer.