Deoxidization phenomenon of iridium oxide (IrO2) film, which has been used as a diffusion barrier for ferroelectric capacitors, is investigated changing the temperature and oxygen pressure during the post process. The deoxidization is well understood by explaining the relationship between the temperature and dissociation pressure according to the thermodynamics. A simple thermodynamic calculation for the deoxidization of IrO2 gives a guideline for successive process condition at which the stability of IrO2 films can be maintained.