(Ba, Sr)TiO3 (BST) thin-film capacitors with Ir bottom electrodes are recognized to have higher polarization and leakage current as compared to those with conventional Pt electrodes. This paper describes a method to improve the leakage current of BST/Ir films by high-density plasma surface treatment using O2·N2O or NH3 gas in an inductively coupled plasma system. It is found that the leakage current density can be reduced by two orders of magnitude using O2 plasma treatment. The dielectric constants were very closely to the reference sample except for NH3 plasma treated sample. The O2 and N2O plasma can enhance the lifetime than 10 years at 1.2 V for the Pt/(Ba,Sr)TiO3/Ir capacitors. It is necessary to trade off the capacitance property for leakage current property.