The surface hardness of the titanium alloy Ti-6Al-4V, widely used as construction material, can be improved by nitrogen implantation. The paper reports on a complex study of the structural mechanisms of hardening and its loss at higher temperatures with different methods (grazing incidence X-ray diffraction (GIXRD), in situ high- temperature diffraction with synchrotron radiation, elastic recoil detection analysis (ERDA), hardness measurements and surface inspection by scanning electron microscopy (SEM)). Plasma immersion ion implantation (PIII) produces a nitrogen profile directly below the surface. The formation of small TiN crystallites is detected, but the hardness increase is only about 40%. Annealing leads to the reduction of the TiN phase and the formation of Ti2N deeper in the material. The in situ X-ray diffraction (XRD) study of the phase formation gives information about the temperature dependence of these changes. The hardness increases after annealing by a factor of 2.5 compared to the unimplanted state. It is explained by precipitation hardening due to the formation of titanium nitrides in different depths of the material.