Nanoparticle size is crucial to drug release behavior and biodistribution in vivo, but few studies have been performed on biodegradable nanoparticles with narrow size distribution. In this note, uniform-sized nanoparticles were prepared by a facile method combining emulsion-solvent removal and premix membrane emulsification for the first time. After preparation of coarse emulsions, additional premix membrane emulsification with very high pressure was occupied to achieve uniform-sized nanodroplets, and nanoparticles were formed by further solidification. Polylactide (PLA) was selected as a model polymer. Several factors played key roles to obtain uniform-sized PLA nanoparticles, including type of organic solvent, the volume ratio of oil phase and external water phase, pore size of the microporous membrane and transmembrane pressure. The coefficient of variation (CV) value of PLA nanoparticles could be controlled below 16.9% under an optimum condition. The novel method also has the advantages of high productivity, simplicity and easy scale-up. The uniform-sized nanoparticles prepared by this novel method have great potentials in drug delivery.