The mechanical properties of natural rubber latex (NRL) modified with styrene-grafted natural rubber (styrene-GNR) latex were investigated. Styrene-GNR was first synthesized via emulsion copolymerization using cumene hydroperoxide/tetraethylene pentamine as an initiator. The styrene-GNR latex produced was mixed with NRL with various latex compounding contents and then prevulcanization was carried out. The mechanical properties and heat, weathering, and ozone resistance of the natural rubber (NR) and styrene-GNR latex compounds were investigated as a function of the grafted NR content. The results indicated that the tensile and tear strength were decreased, whereas Young's modulus and hardness were increased at high content of styrene-GNR. Addition of styrene-GNR improved the resistance of the compounds to heat and weathering ageing. The ozone resistance of the compound containing styrene-GNR is superior to that of the NR-rich compound. The results indicated that NR/styrene-GNR latex compounds maintained good antiageing properties required for outdoor applications. The tensile fracture surface examined by scanning electron microscopy confirmed a shift from ductility failure to brittle with an increase of the styrene-GNR content in the compounds. (C) 2008 Wiley Periodicals, Inc.