Abstract: A binary solvent dispersion(BSD) method has been investigated in preparation of nanoparticles of polylactide(PLA) and poly(DL-lactide-co-glycolide)(PLGA) as functions of polymer molecular weight and monomer ratio in copolymer:L/G ratio=m(PLA)/m(PLGA). The yield, particle size and size distribution of the nanoparticles obtained were examined. Both polymeric nanoparticles could be powderized via lyophilization, leading to the narrow size dirtribution of the particles. The effects of ethanol and acetone on the nanoparticles size and yield have been examined by using of could point titration method. It was found that the yield of nanoparticles increases with increase of ethanol in mixture with acetone and attains a maximum value near the cloud point, whereas the size of nanoparticles decreases with increases of ethanol in the solution with acetone and attains a minimum value near the cloud point. Therefore, the optimum condition for gaining desired yield and size of polymeric particles could be derived form the cloud point experimentation. The yield of nanoparticles was up to 90% with the average size being ranged in 130~180 nm for both PLA and PLGA.