With the catalytic quality improvement of fatty acids, renewable alternative fuels can be produced from waste sources and contribute to sustainable mobility. The aim of this study was to investigate the blending of waste fatty acid in 10, 20, and 30 wt% into unrefined gas oils with different qualities during co-processing in a fixed bed pilot-scale reactor on a commercial, sulphided nickel-molybdenum-alumina catalyst. The effect of the feedstock's fatty acid content and the process parameters on the hydrocarbon composition, sulphur content, aromatic content, and the performance properties of the main products have been investigated. During the experiments, the process parameters were varied in different ranges, such as temperatures of 300-360 degrees C, hydrogen/hydrocarbon volume ratio of 600 N m(3)/m(3), pressure of 50 bar, and liquid hourly space velocity of 1.0-3.0 h(-1). From the results of these experiments, it was determined that fatty acids were completely converted. Under the selected favourable process parameters, which were 10 wt% fatty acid, 360 degrees C, and 1.0 h(-1), high-quality diesel fuel components with high hydrogen contents were produced. The excellent properties of the diesel fuel component, such as 4 mg/kg sulphur content, 1.3 wt% polycyclic aromatic content, and 13.7 wt% hydrogen content promoted cleaner combustion and reduced emissions. Therefore, the obtained renewable diesel fuel-blending component is more environmentally sustainable than conventional diesel fuels.