The formation of metastable phases has a great influence on the mechanical properties of multilayers. A series of AlN/(Ti,Al)N nanoscale multilayers with various modulation periods were synthesized by reactive magnetron sputtering. X-ray diffraction analysis, high resolution transmission microscopy and the microindentation technique were employed to characterize the microstructure and mechanical properties of the multilayers. The results show that metastable c-AlN forms due to the "template" effect of (Ti,Al)N in AlN/(Ti,Al)N multilayers with small modulation I periods, and forms columnar crystals with (Ti,AI)N through epitaxial growth and results in lattice distortion due to the lattice mismatch between c-AlN and (Ti,AI)N. Correspondingly, the hardness and the elastic modulus of the multilayers increase with a decrease of modulation period monotonically and reach the maximal hardness (29 GPa) and elastic modulus (377.8 GPa) at a modulation period of 1.3 nm. (C) 2003 American Vacuum Society.