Nanoparticles of Poly(DL-lactide-co-glycolide) (PLGA) were prepared by dialysis method without surfactant. The size of PLGA nanoparticles prepared from dimethylacetamide (DMAc), dimethylformamide (DMF), and dimethylsulfoxide (DMSO) as an initial solvent was smaller than that of acetone or 1,4-dioxane. Selected initial solvent used to dissolve the copolymer significantly affects the size of nanoparticles. Also, the size of PLGA nanoparticles was changed according to the copolymer composition. It was shown that PLGA nanoparticles have spherical shpaes from the results of scanning electron microscope (SEM) and transmission electron microscope (TEM) observations. From these results was shown the potential that the PLGA nanoparticles could be formed successively by dialysis method without surfactant. The drug-loading contents were also dependent on the copolymer composition and initial feeding amount of the drug. The greater lactide ration on the copolymer composition led to higher drug loading contents. Also, the higher the initial feeding amount of drug, the higher the drug loadging contents. Clonazepam (CNZ) was used as a model drug. CNZ was slowy released in higher lactide ratio in the copolymer composition and in the higher drug loading contents.