The process of high-temperature nitridation (Т = 950°C) of titanium is investigated analytically and experimentally. The role of nitrogen as $\alpha$-stabilizer in forming of nitride layer and diffusion zone, which contains three layers (layer of $\alpha$-phase, layer of $\alpha + \beta$-phase, and layer of $\beta$-phase), is shown. The approximate solution of formulated task is obtained for diffusion of nitrogen in such a heterogeneous medium, taking into account the motion of interfaces. The constants of parabolic growth of layers are calculated. It allows foreseeing the kinetics of their growth and distribution of nitrogen in diffusion zone. The microstructure evolution of diffusion zone (after processing times of 1 h and 5 h), which is caused by the phase—structural transformations during diffusion of nitrogen, is fixed experimentally.