Structural properties of nanocomposite TiAlSiN thin films prepared as monolithic single layer, multilayers and gradient films have been characterized by X-ray diffraction, nano-indentation and atomic force microscopy. The films were deposited onto WC-Co substrates using cathodic arc-plasma. The Al content was varied within the range of Al/Ti=0.5-1.5 and the Si content did not exceed 3.5 at.%. The highest hardness (38-39 GPa) was obtained for samples with Al/Ti = 1.0, associated with more effective combination of solid solution hardening and Hall-Petch effect arising from structure refinement by Al and Si addition. In contrast, the films with Al/Ti = 1.5 showed the lowest hardness values (30-33 GPa) due to the formation of softer wurtzite-like hcp-AlN phase. Formation of layered films slightly improved hardness with respect to monolithic single layer films of comparable chemical composition. Meanwhile, gradient films (Al/Ti = 1.0) showed hardness and modulus values very similar to those obtained for single layer films. Unlike mechanical properties, the formation and propagation of indentation induced cracks appeared more sensitive to film structures (columnar, equiaxed nanograins, ...) as well as to growth morphology (monolithic, layered, gradient). The primary cracks appear for all films beneath the indenter at the contact site. The morphology of these cracks consists of a network of short, discontinuous, irregular cracks in columnar films, which gives rise to straight, well-organized repeated crack in fine grain structures. Such modification of crack patterns is attributed to the role of grain boundary sliding which is more pronounced in coarse columnar films than in nanograined materials. (C) 2003 Elsevier B.V. All rights reserved.