The purposes of this study were to expand the application of a new nanoparticle preparation method, i.e., the modified spontaneous emulsion solvent diffusion (modified-SESD) method, to polylactide (PLA) and poly(DL-lactide-co-glycolide) (PLGA) polymers with different molecular weights, copolymer ratios (L/G ratios) and original sources, and to find the optimal composition of binary organic solvents. Fifteen nanoparticles of different PLA and PLGA polymers were prepared by means of the modified-SESD method, and the yield, particle size and size distribution of these nanoparticles were evaluated. The phase separation points of PLGA and poly(vinyl-alcohol) (PVA) were examined by cloud point titration to clarify the effect of the affinity of solvent used in the system to polymers on nanoparticle productivity. The combination of binary organic solvents, i.e., acetone/alcohol, enabled the production of nanoparticles with good productivity when the L/G ratio was higher than 75/25. The 50/50 PLGA nanoparticles were prepared unsuccessfully in acetone/alcohol, because of poor solubility, but it became successful by replacing the solvents to acetonitrile/alcohol. All of the nanoparticles could be powderized via freeze-drying, and they showed narrow size distributions. The phase diagram study indicated that the balance of the affinity between binary organic solvents and each polymer had a very important role in the productivity of nanoparticles. The productivity was represented a function of chi, a parameter representing the degree of phase separation of PLGA at the actual preparation condition against the ultimate cloud point. The yield of nanoparticles increased with the increase in the chi value and attained almost 100% at more than 0.7 of chi regardless of the difference in binary solvent combination. These results indicated that the optimal composition ratio of binary organic solvents coincided to near the cloud point and the optimal condition of binary organic solvents could be predicted by using their relation curve. It was found that the modified-SESD method was widely applicable to various types of PLGA polymers, by choosing optimal combination of binary organic solvents.