Hard nanocomposite Ti-Si-N films were deposited on 321 stainless steel substrates by direct current (DC) reactive magnetron sputtering using a Ti-Si mosaic target consisting of a Ti plate and Si chips. The composition, microstructure, and mechanical properties were investigated using EDX, XRD, XPS, nano-indentation, and scratch tests. The results indicate that the hardness of the Ti-Si-N film gradually rises with increasing Si contents in the layer until the peak value of 42 GPa appears corresponding to a Si content of 11.2 at.%. The hardness then decreases with further increase in the Si content. XRD and XPS reveal that the hardest Ti-Si-N film consists of fine TiN crystallites (approximately 8 nm in size) in an amorphous Si3N4 matrix. Preferential growth of TiN is indicated by the XRD patterns. All the Ti-Si-N films show high adhesive strength as indicated by the scratch tests. The strengthening mechanism of the nanocomposite films is also discussed.