Polyaniline (PANI) nanoparticles of different sizes were synthesized and embedded into membranes to prepare poly(ether sulfone) (PES)/PANI nanocomposite ultrafiltration membranes. The size and dispersion of PANI nanoparticles were determined using transmission electron microscopy and the dynamic light scattering method. The effects of PANI nanoparticle size and content on membrane characteristics and performance were investigated using Fourier transform infrared spectroscopy, scanning electron microscope, water contact angle measurement, and cross-flow filtration experiments. Overall, PANI nanoparticles that were smaller in size had better dispersion in the casting solution and more uniform distribution in the blended membrane. The nanoparticle size had a significant influence on the modification efficiency of membrane surface pore structure and permeability. The antifouling performance of the membranes was investigated by BSA solution filtration. To evaluate membrane fouling, the filtration resistances were analyzed using the resistance-in-series model.