FIELD : chemistry.SUBSTANCE : present invention relates to a dimer of a cytidine derivative of formula (I), use thereof for producing an antitumour agent, pharmaceutical compositions based thereon and a method for production thereof, which can be used in medicine and pharmaceutical industry (I), where R1 and R2 independently represent C4-alkyl or -(CH2)-Ph; R3 is hydrogen or C4-alkoxycarbonyl; R4 is hydrogen or C4-alkoxycarbonyl; R5 is -(CH2)2-3-. Method involves preparing compounds of formulas (II) and (III), converting a compound of formula (II) into a compound of formula (IV) and reacting compounds (IV) and (III) to obtain a dimer of formula (I) (II) (III) (IV)EFFECT : novel compounds effective for treating malignant solid tumours, pharmaceutical compositions based thereon and an effective method for preparing them are disclosed.8 cl, 4 ex, 9 tbl, 6 dwg