This work study the catalytic co-hydroprocessing of used frying oil (UFO) with atmospheric gas oil (AGO), paying particular attention to the effect of UFO on the hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) efficiency, the products selectivities and its influence on fuel properties. Hydrotreating experiments were performed in a pilot plant for diesel hydrodesulphurization using a commercial catalyst of NiMo/Al2O3, temperature 320-350 degrees C, pressure 5.5 MPa, WHSV 2 h-1, UFO content 20-50 wt% and H-2/feed ratio 500-1200 NL/L. At the operating conditions used, a total conversion of the triglycerides of UFO was obtained with a 96-99% sulfur elimination. This produced a slightly increase of the HDS/HDN rates during the co-processing, without irreversible effects over its activity and important variation in some properties such as cetane index, density 15 degrees C or kinematic viscosity 40 degrees C. The main products obtained were paraffins with 15 and 17 atoms of carbons and light gases such as CO2 and CH4, which imply that the catalyst used stimulates decarboxylation reactions in detriment of hydrodeoxygenation reactions.