The present invention relates to substituted 4'-fluoro-2'-methyl nucleoside derivatives of formula (I). Where R is O-R1, etc. R1 is H, aryl, etc. R´es N (R4) C (R2a) (R2b) C (= 0) OR3 or -OR3, R2a and R2b are H, lower alkyl, etc. R3 is H, lower alkyl, etc. R4 is H, lower alkyl, etc. R5 is H, etc. R6 is OH or F. Base is uracil, cytosine, etc. Preferred compounds: 2'-deoxy-2 ', 4'-difluoro-2'-methyluridine-5' - (O-phenyl-N- (S) -1- (isopropoxycarbonyl) ethyl thiophosphoramidate； 2'-deoxy-2 ' , 4'-Difluoro-2'-methyluridine-5 '- (O-2-naphthyl-N- (S) -1- (isopropoxycarbonyl) ethyl thiophosphoramidate. It also refers to pharmaceutical compositions thereof for the treatment of HCVLa presente invencion se refiere a derivados de nucleosidos 4'-fluoro-2'-metilo sustituidos de formula (I). Donde R es O-R1, etc.R1 es H, arilo, etc. R´es N (R4) C (R2a) (R2b) C (=0) OR3 o -OR3, R2a y R2b son H, alquilo inferior, etc. R3 es H, alquilo inferior, etc. R4 es H, alquilo inferior, etc. R5 es H, etc. R6 es OH o F. Base es uracilo, citosina, etc. Compuestos preferidos: 2'-desoxi-2',4'-difluoro-2'-metiluridina-5'-(O-fenil-N-(S)-1-(isopropoxicarbonil)etil tiofosforamidato； 2'-desoxi-2',4'-difluoro-2'-metiluridina-5'-(O-2-naftil-N-(S)-1-(isopropoxicarbonil)etil tiofosforamidato. Tambien se refiere a composiciones farmaceuticas de los mismos para el tratamiento del HCV
