A combination of high-resolution transmission electron microscopy and x-ray photoelectron spectroscopy are used to establish that Ti-B-N films with different boron concentrations prepared by reactive unbalanced magnetron sputtering exhibit a two-phase nanocomposite microstructure, showing nanocrystalline Ti(N, B) grains embedded in amorphous (TiB2, BN) matrices. Using Monte Carlo simulations and based on a simple model employing a kinetic grain growth theory, we also investigate the effects of the amorphous TiB2-BN phase on the microstructure evolution and grain growth in nanocrystalline-Ti(N, B). Our study demonstrates that the formation of such an amorphous phase at the grain boundary could hinder the growth of Ti(N, B) grains and the mean grain size shows an exponential decay with boron concentration, in good agreement with our experimental observations.