Abstract: OBJECTIVE To synthesize 5-fluorouracil(5-Fu) doped silica(5-Fu/SiO₂) nanoparticles, and to evaluate their parameter of pharmacy and cytotoxicity in vitro. METHODS The 5-Fu/SiO₂ nanoparticles were prepared in the water-in-oil microemulsion, and the administered dose of 5-Fu and the reaction time were optimized to improve the drug loading and entrapment efficiency. The stability and the in vivo release of 5-Fu/SiO₂ were also investigated. The antitumor activities in vitro were evaluated by MTT method. RESULTS With adding 1.33 mg 5-Fu and reacting 24 h, the 5-Fu loading and entrapment efficiency reached maximum yields as 1.03% and 24.77%, respectively. The 5-Fu was released slowly for 48 h, and the size of 5-Fu/SiO₂ nanoparticles had no obvious changing in 7 d. The MTT experiments showed that 5-Fu/SiO₂ nanoparticles had an inhibition on the human hepatic cancer cells, while the SiO₂ nanoparticles had no similar effects. CONCLUSION The 5-Fu/SiO₂ nanoparticles with a good stability and long release is prepared successfully, which provides a new fitting way for developing the 5-Fu dosage forms.