Abstract: In order to improve the properties of the composite,rare earth (RE) modifier was used for the surface modification of glass fibers.The effect of RE concentration in the modifier on tensile properties and bending properties of the glass fiber reinforced unsaturated polyester composites was investigated.The modification effect of the RE modifier was compared to that of the silane coupling agent SG-Si900 (SGS).The fracture surfaces of tensile specimens were analyzed by scanning electron microscopy.The results show that comparing with the silane coupling agent,RE modifier was effective in promoting the interfacial adhesion strength between glass fibers and unsaturated polyester,that could improve the mechanical properties of the composite.The mechanical properties of the composites modified with the RE modifier increased and then decreased with the increase of RE concentration.The maximum tensile and bending loads,which were improved by about 21% and 26% independently compared with those of SGS treated composites,were obtained when the RE concentration was 0.3%.