NEW MATERIAL:A compound expressed by formula I (X is H or F; R is lower alkyl, benzyl, halogen-substituted benzyl, lower acyl or benzoyl).

EXAMPLE: 3'-Azido-5'-methyl-3'-deoxythymidine.

USE: An antitumor agent and antiviral agent.

PREPARATION: For example, 3'-azido-3'-deoxythymidine expressed by formula II or α,α,β-trifluoro-3'-azido-3'-deoxythymidine expressed by formula III is reacted with a halogen compound expressed by the formula R'-Y (R' is lower alkyl, benzyl or halogen-substituted benzyl; Y is Cl, Br or I) in the presence of a base (e.g. sodium hydride) in a solvent (e.g. benzene, THF or acetonitrile) to afford the aimed compound expressed by formula I (group R is R').