To improve the mechanical strength and antifouling properties of poly(ether sulfone) (PES)/sulfonated poly(ether sulfone) (SPES) blend membranes, three kinds of SPES with different molecular weights were attempted as additives. This is the first time that SPES with both high molecular weight (M-w = 141 000 g mol(-1)) and a high degree of sulfonation (DS, 30%) was used for membrane fabrication. It is also the first investigation of the effect of molecular weight of SPES on the properties of the blend membranes. The results show that the compatibility between PES and SPES was very high. The molecular weight of SPES affected the viscosity of the blend solutions but not the cloud point. The membrane morphology, surface chemical compositions, and mechanical strength were investigated. In addition, the surface hydrophilicity, surface charge, pore structures, and antifouling properties of the fabricated membranes were evaluated. The membrane fabricated with a casting solution containing SPES of high molecular weights (M-w = 110 000 and 141?000 g mol(-1)) with a PES/SPES ratio of 9/1 showed high mechanical strength and the superior antifouling potential. This was due to the formation of the thick sponge-like structures, and the hydrophilic and negatively charged surfaces of the membranes.