The present study aims to determine the optimum temperature regime for oxy-nitrocarburizing of AISI D2 steel in order to ensure high mechanical properties of both diffusion and compound zones. The examined samples are quenched and high temperature tempered in advance, thus having a hardness equivalent up to HRc 54-55 before saturation. The vacuum oxy-nitrocarburizing is held at temperatures of 500 ºС, 550 ºС and 590 ºС for 4 hours in a cycling gas flow rate. The proportion of NH3 and CO2 gases used is 90:10 volume fractions. The results of the examination are based on the analysis of X-ray diffraction patterns, cross-section microstructures and measurements of micro-hardness values on the surface and in depth of the diffusion zone. The core hardness of the substrate is also measured after the vacuum process for the purpose of establishing a possible hardness decrease after the oxy-nitrocarburizing process. Conclusions have been drawn in relation to the practice.