Cold work tool steels have been successfully applied for decades. Modern Powder Metallurgy (P/M) techniques are considered important for tool steels in general since high alloy projects could be development. An important example is P/M cold work tool steel X 220 Cr Mo V 13 4. Although tribological properties are quite important to be studied, mechanical properties are also relevant since tools are submitted to high static and dynamic loads. Present work has investigated the effect of gas nitriding process on modulus of rupture, in modern cold work tool steel above mentioned. Nitriding process applied has affected mechanical properties significantly, reducing by 30 % of modulus of rupture decreasing obtained from 4-point bending test. The high C content of those modern steels induces epsilon mono-phase composition in compound layer. Although epsilon nitride, due to its high strength, increases some tribological properties, ductility of those tool steels is clearly affected.