This study investigates direct hydrothermal liquefaction (HTL) of refinery oily-sludge (ROS) to produce bio-oil (BO). Optimum operating conditions of HTL were experimentally determined by response surface methodology (RSM). Experiments were implemented in a tube-bomb reactor at various temperatures (275-350 degrees C), reaction time (30-75 min) and ROS content (10-40 wt%) in feed. BO yield reached to 45.52 wt% on dry ash-free basis at optimum conditions (290 degrees C, 65 min and 16 wt% of ROS). GC/MS analysis showed that BO mainly contained aliphatic, saturated and unsaturated fatty acids, monoaromatic and polyaromatic compounds. Moreover, it was found that by increasing temperature, small compounds were converted into heavier molecules via repolymerization reactions. With increasing reaction time at low temperatures, the amount of nitrogen-containing compounds increased, emphasizing the undesired effect of very long reaction time. Results also indicated that catalytic upgrading is required for dehydrogenation, oxygen removal and new C-H bonds creation.