As an alternative resin to conventional synthetic unsaturated polyesters (UPEs), epoxidized maleinated castor oil (EMACO) was synthesized in two steps. For this purpose, castor oil was reacted with maleic anhydride at 70 degrees C to obtain maleinated castor oil (MACO). Then, epoxidation of MACO was carried out by using a mixture of formic acid and hydrogen peroxide at 0-5 degrees C. Then, the free carboxyl groups of the synthesized EMACO were further reacted with free epoxide groups of EMACO at 90 degrees C. At the end of the reaction, an unsaturated polyester precursor-prepolymer was obtained (P-EMACO). FTIR and H-1 NMR spectroscopic techniques were used to characterize the monomers synthesized. The P-EMACO was then mixed with styrene and cross-linked in the presence of AIBN at 50 degrees C. Thermal and mechanical properties of the final cross-linked product were investigated by thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA) techniques. The degradation onset temperature of the material at which cross-linked X-EMACO loses 5% of its weight was found to be 209 degrees C. Its dynamic T-g and storagemodulus at 25 degrees C were determined as 72 degrees C and 1.08 GPa, respectively. These results are higher than some of the different oil based polymers reported in the literature.