Abstract: In this study, a new dosage form for antitumor agents—— nanocapsules was prepared from biodegradable butylcyanoacrylate (BCA). Using 5- fluorouracil (5-Fu )as the model drug, scanning electron microscope pictrues show that the maximun diameter of nanocapsules particles is about 338 nm. A new gel chromatography method is established for the determination of encapsulation efficiency in nanocapsules. The encapsulation efficiency of 5-Fu is 40.9%. LD₅₀ of BCA is 267.7 mg/kg for intravenous injection in mice. The antitumor activities of 5- Fu nanocapsules on S₁₈₀, HepS and EAC were found to be higher than that of free 5- Fu.