In this article results of an investigation into effects of multilayer structures of compositionally modulated Ti/TiN films on their hardness is presented. Compositionally modulated Ti/TiN multilayer films were deposited onto borosilicate glass substrates by a reactive gas flow rate modulation magnetron sputtering method using a combination of a Ti metal target and an Ar-N-2 mixture discharge gas. In this method, a N-2 gas flow rate was changed by a computer-controlled mass flow controller to produce films with a varying composition distribution. The film hardness was measured by nanoindentation. The film composition depth profiles were estimated by Auger electron spectroscopy and x-ray photoelectron spectroscopy. A TiN single layer him was also deposited to compare its hardness with those of the Ti/TiN multilayer films. Modulation periods examined in this study were 10, 15, 20, and 40 nm. It was found that the film with a modulation period of 20 nm showed a maximum hardness of 29 Cpa and that the films with modulation periods of 15 and 40 nm yielded a higher hardness value than that of the single layer TiN film of 15 GPa. The results obtained in this study indicate that the hardness of compositionally modulated Ti/TiN multilayer films strongly depend on modulation frequency.