Abstract: The reaction between technetium and hydrazine in nit ric acid medium is studied by the titration of the concentration of hydrazine and pursuing the concentration of heptavalent technetium along with time. It is approved that the reaction between technetium and hydrazine includes a inducing period, a fast reaction stage and the termination of the reaction. The inducing period is relevant to the initial concentration of heptavalent technetium, temperature and acidity and no relationship between its length and the initial concentration of hydrazine. It is tetravalent technetium existing as TcO 2 and TcO 2+ who catalyzes the oxidation of hydrazine. In fast reaction stage, the kinetics equation can be written as follows: -d c (N 2H 4)/d t = kc (N 2H 4)· c -1 (H +), which is no relation with the concentration of heptavalent technetium. The activation energy of the stage is 91.8 kJ/mol. In the termination period some hydrazine remains in the system and the concentration of heptavalent technetium begins to rise.