Chromium aluminum oxynitride (Cr-Al-N-O) thin films have been prepared by pulsed laser deposition. Experiments were carried out by changing the surface area ratio of targets [S-R = S-AlN/(S-Cr2N+S-AlN)] from 0% to 100%. The composition of the thin film prepared at S-R=75% was determined to be Cr0.11Al0.39N0.25O0.25 by Rutherford backscattering spectroscopy. From the result of grazing angle x-ray diffractometry, the phase transition of the Cr-Al-N-O thin films was clarified to occur, when the aluminum content in cations (x) ranged from 70 to 90 at. %. In addition, the thin films having composition close to the solubility limit were observed by transmission electron microscopy to contain chromium nitride particulates below 200 nm in diameter. The Cr-Al-N-O thin film with x = 25 at. %, which was the hardest over the whole x range, had Vickers hardness above 4000. In this thin film, nanometer-sized crystallites were found in, the grains. Moreover, the elastic modulus was shown to be higher than those of other Cr-Al-N-O thin films. (C) 2003 American Vacuum Society.