With the aim of diminishing the toxicity of 5-fluorouridine (1) and obtaining biologically active derivatives of 1, various kinds of 5'-O-acyl-8-fluorouridines 2 were synthesized. The antitumor activity of the compounds against L-1210 leukemia in mice was examined. The 5'-O-heptanoyl derivative 2h showed the highest antitumor activity.