LaNiO3 (LNO) conductive thin films with different thicknesses were deposited on SrTiO3 (100) substrates in different oxygen pressures. Effects of oxygen pressure on microstructure of LNO conductive film have been studied by in situ reflection high energy electron diffraction (RHEED) and ex situ x-ray photoelectron spectroscopy (XPS). In the relatively low oxygen pressure, LNO film displays spotty RHEED pattern. When the thickness increases up to a critical value, about 30 nm, the spotty RHEED pattern gradually changes to streaky pattern, and the RHEED oscillation curve appears. The streaky RHEED pattern of LNO film deposited in the relatively high oxygen pressure can be observed at the initial growth. With pumping the oxygen pressure to a relatively low value, the RHEED pattern gradually changes to spotty one. When the oxygen pressure is increased back to a high value, the RHEED pattern changes to streaky one again. This RHEED pattern transformation induced by the oxygen pressure is reversible. Ex situ XPS results indicate that the element Ni of LNO film deposited in the relatively low oxygen pressure with a thickness below the critical value exists in the form as W, while as W, in the relatively high oxygen pressure. When the thickness increases to the critical value, the chemical valence of element Ni in the top layers of LNO film deposited in the relatively low oxygen pressure is +3. A mechanism of effects of oxygen pressure on rnicrostructure of LaNiO3 conductive thin film is given. This sensitivity to oxygen for LNO thin film can be used in sensor devices. (c) 2006 American Vacuum Society.