A series of novel 4-chlorophenyl N-alkyl phosphoramidates of 3'-azido-2',3'-dideoxy-5-fluorouridine (12-21) were synthesized by means of phosphorylation of 3'-azido-2',3'-dideoxy-5-fluorouridine (4) with 4-chlorophenyl phosphoroditriazolide (10) followed by a reaction with the appropriate amine. The synthesized phosphoramidates (12-21) were evaluated for their cytotoxic activity in three human cancer cell lines: cervical (HeLa), oral (KB) and breast (MCF-7) using the sulforhodamine B (SRB) assay. The highest activity in all the investigated cancer cells was displayed by phosphoramidate 13 with the N-ethyl substituent and its activity was much higher than that of the parent nucleoside. Also phosphoramidate 17 with the N-propargyl substituent exhibited good activity in all the used cell lines.