We report the improved mechanical properties of three kinds of nitriding steels modified by titanium diffusion either by depositing titanium by PVD and hot-pressing the steel with titanium sheets hot pressed at 950 degrees C. The steels used were AISI series: 1045, O-1 and H-13. A mixed layer of Ti and N compounds was generated by a post treatment. Hardness measurements showed an increase of approximately two times in hardness with respect to the annealed condition. The results of fracture toughness tests indicated excellent strengthening of the steels. The role of composition and microstructure on strengthening are discussed.