In the present work, bio-oil derived from the catalytic pyrolysis of sewage sludge has been blended in small amounts with sunflower biodiesel with the aim of evaluating its potential as a novel, low-cost, and renewable biodiesel additive that could replace costly commercial biodiesel antioxidants normally used to date. The effect of blending small amounts of bio-oil with sunflower biodiesel on the biodiesel properties (oxidation stability, cold flow properties, flash point, and viscosity) has been analyzed. Furthermore, apart from studying the effect of adding low bio-oil concentrations (0.1, 1.8, and 3.5 mass %), the effect of other operating conditions, specifically the temperature (278-333 K) and mixing time (5-60 min), during the bio-oil and biodiesel blend preparations has also been analyzed. With regard the oxidation stability, blends prepared adding 3.5% mass fraction of bio-oil complied with the limits imposed by the ASTM D6751 and EN 14214 standards. Blending sewage sludge bio-oil and sunflower biodiesel did not result in an enhancement of the biodiesel cold flow properties in the studied range, while the flash point of these blends was lower than that of pure sunflower biodiesel. The viscosity was barely affected in all cases. The oxidation stability enhancement achieved by the addition of bio-oil obtained from sewage sludge catalytic pyrolysis was higher than the enhancement obtained with bio-oil from non-catalytic pyrolysis.