Unsaturated polyester resin (UPR) is a thermoset-type polymer, and it is widely used in the fiber-reinforced plastic industry. This can be attributed to its low cost, ease of processing and combination with reinforcements, rapid cure, excellent dimensional stability, and a wide variety of grades available. This study was conducted to investigate the effect of grafted copolymers on the properties of UPR. Testing included viscosity, gel time, curing temperature, hardness, tensile strength, and compressive strength. The grafted copolymers were based on polypropylene glycol (PPG) grafted with styrene (St) at different St ratios (65/15, 65/25, 65/35). The grafted copolymers were prepared by free-radical polymerization in the presence of potassium persulphate as an initiator. The results showed that significant improvement in the physical and mechanical properties of UPR due to the modification of UPR by the prepared grafted copolymer. It was found that the physical and mechanical properties of UPR enhanced with increasing the ratio of St in the grafted copolymer and the concentration of grafted copolymers. This is attributed to the high rigidity of the grafted copolymer containing the highest styrene content.