Abstract: In 1B tank of APOR, where monomethylhydrazine(MMH)-dimethylhydroxyla-mine(DMHAN) are adopted as reductants, most of technetium flows into 1BP for unknown reason. The phenomena was explained through the reaction kinetics between Tc(Ⅶ) and MMH or DMHAN, the results of multi-stage extraction experiments and continuous mixer-settler experiments. Experiment results show that technetium is reduced to unextractable low valent state by MMH, and the reduction reaction is greatly accelerated in the presence of uranium. The reduction of Tc is probably the factor to dominate the distribution of technetium in 1B tank. The difference between the distribution of technetium in multi-stage extraction experiment and mixer-settler experiment was also discussed in the present paper.