Graft copolymers of natural rubber and polystyrene were synthesized by free-radical grafting of styrene monomer onto natural rubber in latex form. The obtained graft copolymers and unsaturated polyester (UPE) resin were mixed and cast at room temperature using methyl ethyl ketone peroxide as an initiator and Co-octoate as an accelerator. The samples prepared from ungrafted natural rubbers exhibited the aggregation of the rubber component, whereas the samples prepared from grafted natural rubbers showed good dispersion of the rubber component in a glassy matrix of UPE resin. It was found that the amount of polystyrene grafted onto natural rubber and the graft copolymer content in polymer blend significantly affect the mechanical properties of the blend samples. An increase in the amount of hard and brittle polystyrene in glassy matrix of UPE resin overshadowed the impact-absorbing ability of the rubber component, causing the impact strength of the blend samples to be lower than that of pure UPE resin. On the other hand, an increase in easily elongated un-crosslinked rubber molecules, as the graft copolymer content in blend samples increased, resulted in a decrease in their flexural strength. (C) 2004 Wiley Periodicals, Inc.