Nano-structured TiN/TiBN multilayer thin films were deposited onto unheated Si(100) substrates by reactive unbalanced dc-magnetron sputtering in an Ar-N2 gas mixture at a pulsed-bias voltage of –60 V. The effects of the bilayer thickness (Λ = 1.8-7.7 nm) on microstructures and mechanical properties have been analyzed using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and microindentation measurements. Microstructure studies revealed that the TiN layers were fcc B1-NaCl structure comprising of (111)- and (200)-oriented grains depending on Λ, while the TiBN layers were amorphous. Significant relationships were found between hardness (H) and Λ. A maximum hardness of ~30 GPa was observed in a multilayer film with = 1.8 nm. The possible hardness enhancement mechanism was also discussed.