A process for producing a 4-O-sulfonyluracil derivative having the formula ##STR9## wherein R1 is a hydrogen atom, a halogen atom, an alkyl group or an alkoxyl group, R2 is a glycosyl group whose hydroxyl groups are protected, and R3 SO2 is an organic sulfonyl group which comprises reacting an uracil derivative having the formula ##STR10## wherein R1 and R2 are as defined above with an organic sul