PURPOSE: To directly synthesize a semisynthetic glycosaminoglycan in one reaction by reacting a glycosaminoglycan having a heparin or heparan structure with a nucleophilic agent in the presence of an inorganic or organic salt in an aqueous solution.

CONSTITUTION: A glycosaminoglycan having a heparin or heparan structure represented by formula I is reacted with a nucleophilic agent for 2-120 hr in the presence of an inorganic or organic base in an aqueous solution to produce a semisynthetic glycosaminoglycan having a heparin or heparan structure represented by formula II. In the formulae, m and n are each 1-100; R is H or a sulfuric acid residue (SO3-); p is an integer except 0; p+q=m, m and n are each 1-100; and -X is a nucleophilic group. According to the above process, a semisynthetic glycosaminoglycan having a heparin or heparan structure modified in position 2 of α-L-iduronic-2-O-sulfate acid can be directly synthesized in one reaction not via a route of a semisynthetic glycosaminoglycan of an epoxide.