Orthogonally protected disaccharide building blocks for synthesis of heparin oligosaccharide. The disaccharide building block has a formula (I), in which L is a leaving group, P.sup.1, P.sup.2, P.sup.3 and are different, and of them P.sup.1 is an ester-type protecting group, P.sup.2 is a hydroxyl protecting group that could be oxidized to a carboxylic acid, P.sup.3 is a hydroxyl protecting group, and P.sup.4 is a hydroxyl protecting group which allows chemoslective deprotection with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Acting as an elongation unit, the disaccharide building block of formula (I) may react with a starting unit of formula (II) to synthesize a heparin oligosaccharide. ##STR00001##