Abstract: Objective: To investigate the slow- release characteristics and antitumor effects of the bio- soluble 5- fluorouracil- loaded immunological nanoparticles, prepared by attaching anti- VEGF mon-oclonal antibodies to 5- fluorouracil- loaded polylactic acid nanoparticles (5- FU- PA- NPs), in order to assess their anti- tumor activity in vitro. Methods: Constant temperature vibration dialysis and MTT colorimetric assays were used. Results: The 5- FU- PA- NPs showed appropriate extended release with a half- life of 10.4 days. There was a positive correlation between the anti- tumor activity and the reaction time and dose of the controlled release. Conclusion: The 5- FU- PA- NPs show a uniform drug deposition, and the therapeutic effect is not affected by preparation and metabolic breakdown of the drug. The 5- FU- PA- NPs have a well- controlled release which can maintain effective tumor- killing activity for an extended period.