Ti-Al-Si-N films with various composition ratios were deposited by a specially designed cosputtering system with three magnetron cathodes by controlling the impressed power ratios on each cathode independently. All the films were deposited on Si wafers at 300 degrees C under a total gas pressure of 0.5 Pa with a nitrogen flow ratio [N-2/(N-2+Ar)] of 70% using Ti, Al, and Si targets. Hardness and Young's modulus normal to the surface of the Ti-Al-Si-N films were analyzed by nanoindentation equipped with a Berkovich diamond indenter. It was clarified that the hardest condition of the films with different Ti/Al ratios could be obtained when the Si content was about 10-20 at. %, whereas the maximum values of the hardness did not depend on the Al content. Nanocomposite structures where Ti-Al-N or Ti-Al-Si-N nanocrystallites were embedded in an amorphous Si3N4 matrix were clearly observed by transmission electron microscopy which must contribute largely on the hardening of the films. The size of the nanocrystallites for the hardest film was about 25 nm. On the other hand, the solid solution of Al atoms into TiNx was not so effective in the Ti-Al-Si-N films. The hardest film of the Ti-Al-Si-N system in this study was Ti0.77Al0.06Si0.17Nx, with a hardness of 34 GPa. (c) 2007 American Vacuum Society.