This study focuses on evaluating the effect of residual lipid co-hydroprocessing with a heavy petroleum fraction, towards the production of a hybrid diesel fuel of increased sustainability. Firstly, the effect of Waste Cooking Oil (WCO) co-hydroprocessing with Heavy Atmospheric Gas Oil (HAGO) at different relative ratios (100/0, 95/5, 90/10, 85/15, 80/20 and 70/30 v/v) was evaluated in terms of diesel quality. The experimental results showed explicitly that the addition of WCO did not limit any hydroprocessing reactions targeted. Particularly WCO integration led to a significant diesel content increase which reached 13% for the highest WCO content feedstock (30% v/v), while increasing hydrogen consumption. The study showed that co-hydroprocessing of residual lipids within a refinery can provide an effective and no-risk pathway for integrating residual liquid biomass in transportation fuels, without significant capital investments, while the addition of 5-10% v/v WCO in the hydroprocessing feedstock is considered optimal in terms of H-2 additional requirements (similar to 6.5%), product quality and WCO availability. (C) 2014 Elsevier Ltd. All rights reserved.