Functional graded Ti-Al-Si-N-O nanocomposite films were deposited onto WC-Co substrate by a filtered arc ion plating system using TiAl and TiSi composite targets under N₂/Ar atmosphere. XRD and XPS analyses revealed that the synthesized Ti-Al-Si-N-O films were nanocomposite consisting of nanosized (Ti, Al, Si)N crystallites embedded in an amorphous Si₃N₄/SiO₂ matrix. The hardness of the Ti-Al-Si-N-O films exhibited the maximum hardness values of ~47 GPa at a Si content of ~5.63 at.% due to the microstructural change to a nanocomposite as well as the solid-solution hardening. Besides, Ti-Al-Si-N-O film with Si content of around 5.63 at.% also showed perfect adhesive strength value of 105.3 N. These excellent mechanical properties of Ti-Al-Si-N-O films could be help to improve the performance of machining tools and cutting tools with application of the film. A comparative study on microstructural characteristics among Ti-Al-Si-N-O films with various Si contents is reported in this paper.