A detailed kinetic study of ligand-exchange reaction between ^99mTc-GH and EC was carried out. The rate constants (k) of ligand-exchange reaction at different EC concentrations and different pH values were calculated. The k values (L·mol^-1·min^-1) were 3.7×10³, 4.1×10³, 3.9×10³ and 3.9×10³ at EC concentrations (μmol·L^-1) of 559.7, 279.9, 186.6 and 55.97, respectively, while k values were 3.7×10³, 3.4×10³, 1.9×10³ and 3.1×10² when pH values were 10, 9, 8 and 7, respectively, and k had an increase of the order of magnitude from pH 7 to pH 8. It demonstrated that k of the ligand exchange reaction was pH dependent and pH value of the reaction solution must be equal to or little higher than 8 in order to make the labeling yield of ^99mTc-EC higher than 90%.