Ir films were prepared by metal organic chemical vapor deposition (MOCVD), and their electrochemical properties as the electrode for oxygen gas sensors were investigated. The Ir films consisted of It granules (1 to 3nm in diameter) and amorphous carbon surrounding each Ir granules. The charge transfer at the nano-graular Ir film electrode/ zirconia solid electrolyte/gas interface was about 100 times greater than that for other platinum electrodes below 500°C. The emf values of the oxygen cell constructed from the nano-granular Ir film electrode and zirconia solid electrolyte showed the Nernstian theoretical values even at 300°C.