Abstract: Titanium nitride is attractive for nanophotonics applications due to its gold-like but tunable optical properties. The surface morphology, shape, and size are effective parameters that can be used to control the plasmonic properties of titanium nitride. However, the grain size effect of titanium nitride thin films on the localized surface plasmon resonance properties has not been intensively studied yet. In this paper, we prepared a series of titanium nitride thin films with different grain sizes by adjusting the N₂ concentration and film thickness during magnetron sputtering, in order to regulate the plasmonic properties of the films. As the N₂ concentration increases during the sputtering process, the composition of the deposited films becomes closer to stoichiometric titanium nitride, and the grain size decreases from 18.3 nm to 13.2 nm. Moreover, with the decrease of film thickness to 43 nm, the grain size further reduces to 7 nm. The investigation on the dielectric function using spectroscopic ellipsometry indicates that the decreased grain size plays a role in the enhancement of the plasmonic properties.