Abstract: New nucleoside derivatives possessing strong anti-tumor activity with low toxicity, represented by the general formula: ##STR1## wherein (A-CO-- is a residue of a saturated straight or branched chain fatty acid, B is a nitrogen-containing group, Q is a substituent of the fatty acid, Z and Z' each is H or OH with the proviso that both of Z and Z' are not OH, and n is zero or an integer of at least 1, as well as physiologically acceptable salts thereof. These nucleoside derivatives are prepared by introducing the nitrogen-containing acyl group directly in one step or indirectly in two steps into the 5'-position of 5-fluorouridine, 2'-deoxy-5-fluorouridine or 1-.beta.-D-arabinofuranosyl-5-fluorouracil and splitting off any protective group and optionally converting the free compound into a physiologically acceptable salt thereof or vice versa.