The tensile stress-relaxation characteristics at room temperature of gum NR vulcanizates containing 10-20 phr of phosphorylated cashew nut shell liquid (PCNSL) prepolymer were studied at various strain rates and strain levels, in comparison with that containing the same dosage of a-ethyl hexyl diphenyl phosphate and the unmodified sample. Modification of NR with 10-15 phr of PCNSL resulted in improvements in tensile properties along with a lower degree of stress relaxation at the higher strain rates and strain levels.