An even and compact iridium layer was prepared on a platinum substrate by electrodeposition under a constant voltage in an ionic liquid of 1-butyl-3-methylimidazolium chloride (BMIC). The surface morphology and composition of the iridium layer were characterized by scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). Results show that iridium layer can’t be prepared in pure BMIC. Adidtion of ethylene glycol and an evaluated temperature could effectively improve the solubility of IrCl3 in BMIC, which is useful in preparing iridium layer. Iridium layer is obtained at the right voltage and no iridium is got on platinum under a low voltage. When the voltage is too high, the iridium layer becomes loose. When the molar ratio of BMIC to ethylene glycol increases, the surface of the iridium layer becomes rough. Iridium layer can not be achieved at lower temperature while at higher temperature a larger crack appears on the iridium layer