The effects of molecular structure on the redox properties of the organic electroluminescent materials (Ir$(ppy)_3$ Ir$(m-ppy)_3$ Ir$(p-toly)_3$) were studied using cyclic voltammetry and spectroscopy. These iridium complexes show reversible oxidation and reduction on the electrode, which produce the symmetric cyclic voltammogram. It indicates that these materials are very stable under repetitive oxidation/reduction cycles. The electrochemically determined ionization potentia/electron affinity values are 5.4OeV/3.02eV for Ir$(ppy)_3$, 5.36eV/2.96eV for Ir$(m-ppy)_3$, and 5.35eV/2.97eV for Ir$(p-toly)_3$ from the SCE(Standard Calomel Electrode). The electrically determined band gaps are 2.38eV (521nm), Ir$(ppy)_3$, 2.4OeV (517nm), Ir$(m-ppy)_3$, and 2.38eV (521nm). Ir$(p-toly)_3$, which are similar with the optical band gaps. The position of methyl group on 2-phenylpyridine (ppy) effects do not influence much on the ionization potential, electron affinity, and band gap of Ir$(ppy)_3$ derivatives.