Antitumor-active derivatives of 5-fluorouracil were prepared via a new method by introducing an acyloxymethyl group at the 1-, 3-, or 1, 3-position (s). These derivatives were obtained by condensing 1, 3-bis (hydroxymethyl) -5-fluorouracil with various short-/long-chain carboxylic acids or their derivatives, in the presence of dicyclohexylcarbodiimide and a catalytic amount of N, N-dimethylaminopyridine. Some of the derivatives showed strong antitumor activity against the leukemia L1210 system when administered orally.