An attempt to prepare graft copolymer of natural rubber, NR and poly(stearyl methacrylate), PSMA, was made to improve mechanical properties of NR, since the grafted PSMA was composed of crystallizable stearyl groups as a nucleating agent and methacrylate units. The graft-copolymerization of stearyl methacrylate (SMA) onto deproteinized natural rubber (DPNR) was performed in latex stage with tertbutylhydroperoxide (TBHP)/tetraethylenepentamine (TEPA) as an initiator, after purifying the rubber with urea and sodium dodecyl sulphate followed by centrifugation. The optimal conditions for the graft-copolymerization were determined as follows: 3.3 x 10(-2) mol/kg-rubber of initiator, 1.5 mol/kg-rubber of monomer and 30 w/w% of dry rubber content (DRC). The conversion and grafting efficiency of SMA were 95 mol% and 60 mol%, respectively. After graft-copolymerization, the melting temperature of PSMA increased from 304.1 K to 306.2 K. The morphology showed that the DPNR particles, which were about 1 mm in average diameter, were well dispersed in PSMA nanomatrix. The stress at break increased about three times, i.e. 13 MPa, as high as that of DPNR. The increase in the mechanical properties was promoted by the nucleating effect of stearyl group of PSMA. (C) 2016 Elsevier Ltd. All rights reserved.