The diffusion saturation of (α + β) titanium alloy by nitrogen under rarefied medium taking into account the peculiarities of interaction both at the interface and in the metal core is modeled. The corresponding diffusion problem using the diffusion equations in the medium consisting two families of diffusion paths and non-stationary boundary condition at interface is analytically solved. The effect of the temperature and time parameters, volumetric quantity of β- phase on the kinetics of nitriding is estimated. It is established that with the increase of isothermal exposure temperature and quantity of β-phase the surface saturation by nitrogen decreases. It is caused by more intensive nitrogen penetration into bulk and formation of a deeper diffusion zone.