Glycidyl 3-pentadecenylphenyl ether was synthesized from the reaction of cardanol and epichlorohydrin in the presence of sodium hydroxide and characterized by infrared spectroscopy and nuclear magnetic resonance spectroscopy. The glycidyl 3-pentadecenylphenyl ether (GPPE) modified natural rubber was prepared by direct mixing the natural rubber latex, GPPE, and additives in a ball mill. The standard vulcanized sheets were prepared for thermal and mechanical properties testing. The effect of GPPE concentration, type and concentration of surfactant, and mixing time on the mechanical properties of vulcanized sheets were studied. Thermogravimetric analysis and thermal aging were used to determine the thermal properties of vulcanized sheets. The dispersion of GPPE and additives were investigated by optical microscope and scanning electron microscope. The percent crosslinking of vulcanized sheets was also studied by soxhlet extraction. The results of mechanical properties indicated that the suitable formulations of the GPPE modified natural rubber were 100 parts by dry weight of concentrated rubber latex, 5 phr zinc oxide, 2 phr sulfur, 2 phr stearic acid, 1 phr MBT, 2 phr GPPE, and 1.5 phr Tergitol NP10. The rubber product had thermal decomposition temperature at 393.13 ํC. The percent crosslinking of rubber containing of 2 phr GPPE were 95%. This rubber product had mechanical properties better than some other CNSL modified natural rubbers and suitable for preparing some rubber products.