Doxorubicin (DOX)-conjugated poly(DL-lactide-co-glycolide)(PLGA) was synthesized and nanoparticles were prepared without the use of a surfactant. The DOX content in the nanoparticles ranged from 1.2 to 4.75% (w/w). The particle size of the DOX-conjugated PLGA (DOX-LG) nanoparticles increased when a higher molecular weight (M.W.) of polymer was used. TEM revealed the nanoparticles to have spherical shapes with a size of approximately 100 similar to 200 nm. The drug was released continuously from the nanoparticles over a one month period. In particular, the overall drug release from the nanoparticles prepared with a higher M.W. was significantly faster than that from nanoparticles with a lower M.W. The cytotoxicity test in vitro using CT26 colon carcinoma cells showed that the free DOX had higher cytotoxicity than DOX-LG. These results suggest that DOX was released slowly from nanoparticles and was exposed to the tumor cells. Moreover, DOX-LG might show low cytotoxicity due to these properties of nanoparticles.