Abstract: In this paper, the epoxidized soybean oil (ESO) was prepared by solvent-free method. The acrylated epoxidized soybean oil (AESO) was obtained by the reaction of ring opening of ESO using acrylic acid as ring opener. The blend materials were made by mixing unsaturated polyester resin (UPR) and AESO with the mass ratio of 5%, 10%, 15% or 20%. The properties of toughened unsaturated polyester resin were characterized by fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermal gravity analysis (TG) and mechanical testing. The results indicated that the glass transition temperature and tensile strength of toughened unsaturated polyester resin decreased while elongation at break and impact strength increased with increasing of soybean oil content. The results showed that the prepared soybean oil is an excellent flexibilizer for unsaturated polyester resin.