Hot work tool steels are widely used as thermal and mechanical highly stressed hot work tools for pressure die casting moulds, die insert, steel forging and extrusion tools for metal alloys and plastics. Plasma-nitriding has proved to increase the lifetime of such tools by mechanism of the precipitation hardening. To optimize the mechanical properties, it is indispensable to determine and understand the mechanisms which occur in the near-surface zone during nitriding. The influence of silicon content on the formation of nitride precipitations in the near surface zone during nitriding and their mechanical properties were investigated in comparison of a low and high silicon grade steel of the type X 38 CrMoV 5 1 (AlSI H13). Nitrogen and carbon profiles obtained by glow discharge optical emission spectroscopy (GDOES) and studies of the nanostructure by small angle X-ray scattering (SAXS). Microhardness and residual stress profiles show the correlation of the microstructure with mechanical properties. Ductility investigations and bending tests of the nitrding samples have been carried. The background of this thesis includes a broad review of state of the art about the plasma-nitriding of tool steels.