Abstract: At 60℃ and in the presence of benzoyl peroxide, fulvic acid has been grafted onto natural rubber through heterogeneous phase polymerization. Tbe grafted product is soluble in organic solvents in common with the original rubber. A new absorption band (1130 cm^-1) appeared in the IR spectrum of the grafted polymer, indicating the presence of C-O-C bond as the grafting site. The T_g of the grafted polymer measured by DSC methed is -53℃.