Thermo-diffusion coatings containing Nitrogen, Carbon and Niobium (N+C+Nb) on AISI D2 steel have been carried out by an initial tufftriding process followed by saturation with Niobium. The properties of the diffusion layer, namely microstructure, phase composition and micro-hardness of the Niobium carbonitride layer, have been studied. The influence of treatment time of Niobizing on the thickness of the metallized layer and its phase composition has been studied. Nitriding treatment was performed at 575°C for 2 h. Then, the Niobizing treatment was performed by pack method in the powder mixture consisting of ferro-Niobium, ammonium chloride and alumina at 1000°C for 1–4 h. The phases formed on the Niobium carbonitride coated steel were NbN and NbC, confirmed by X-ray diffraction (XRD) analysis. The longer the treatment times, the thicker the Niobium carbonitride layer became. The thickness of Niobium carbonitride layer was changing between 6.53 3m and 17.45 3m, depending on treatment time and temperature. The microhardness of Niobium carbonitride layer formed on the AISI D2 steel was changing between 2132±203 and 2814±245 HV0.01 from surface to interior.