Abstract: The terephthalic type unsaturated polyesters resin(UPR), prepared with dimethyl terephthalate(DMT), maleic anhydride(MA) and propylene glycol(PG), was reinforced with the surface-modified MgO powder to prepare MgO/UPR composite through mechanical blending. The dispersion of surface-modified MgO powder in the UPR matrix, the effect of the mass fraction of MgO on the dynamic thermal mechanical properties, heat resistance and hardness of MgO/UPR composites were investigated. XRD, FTIR and SEM results illustrate that the surface of MgO is coated with a layer of organic functional groups and disperses well in UPR after being modified by sodium stearate. The results of dynamic mechanical analysis (DMA) show when the mass fraction of MgO is 9%, the MgO/UPR composite has the best dynamic mechanical properties, heat resistance, and slow decrease of storage modulus with increasing temperature. The glass transition temperature(T_g) reaches 191 ℃ which is 66 ℃ higher than that of the pure UPR. The hardness analyses indicate MgO/UPR composite has the maximum barcol hardness HBa 44 when the mass fraction of MgO is 4%, which is 18.9% higher than that of pure UPR.