A series of six novel 5-fluorouracil derivatives 1-6 were synthesized and their structures confirmed by (1)H- and (13)C-NMR, MS and elemental analysis. The preliminary in vitro antitumor activities against B16, K562 and CHO cells and the in vivo inhibitions of liver cancer H(22) demonstrated that some of these compounds effectively inhibit the growth of tumor cells, but the in vivo trials in mice revealed that the compounds also exhibited serious liver and lung tissue toxicity. The hydrolysis experiments indicated that this type of compound did not readily liberate 5-fluorouracil, as expected.