Natural rubber (NR) has been modified with 10 phr each of phosphorylated cashew nut shell liquid (PCNSL) prepolymer and an aromatic oil (spindle oil) in a typical semi-efficient vulcanization (SEV) system. Despite the lower chemical crosslink density, the PCNSL modified NR vulcanizate showed higher tensile strength, elongation at break, thermal stability, and resistances to fatigue failure and thermo-oxidative decomposition, as compared to the vulcanizate containing the same dosage of spindle oil.