Density functional theory, employing B3LYP/DZVP and B3LYP/6-31G*(LANL2DZ for Tc), has been used to investigate the interconversion mechanism between formal TcO³⁺ and TcO₂ ⁺ core of ^99mTc labeled amine-oxime (AO) complex, in which two water molecules have been used to simulate the possible interconversion process. The obtained results indicate that the length of amine-amine hydrocarbon backbone of AO ligand has a significant influence on the stabilities of formal TcO³⁺ and TcO₂ ⁺ complex. The interconversion process between TcO–BnAO and TcO₂–BnAO has been amply discussed, which releases the useful information for the further investigation of the structure and hypoxic mechanism of ^99mTc-HL91.