Vegetable oils as energy source can either be used in their pure form or as bio-diesels after transesterification. In the present study, performance of a hollow cone pressure atomizer and a twin fluid atomizer with sunflower straight vegetable oil and its ester (bio-diesel) has been compared. The pressure drop (Delta P) of liquid across the atomizer and the liquid flow rate (Q) have been measured for different liquid temperatures and atomizer liquid flow number (FN) and coefficient of discharge (C-d) are calculated. The spray cone angles have been evaluated from optical images of the sprays at different conditions. Though, the flow number is found to increase upon transesterification in case of the twin fluid atomizer, an opposite trend is observed for the hollow cone pressure atomizer. It shows that less pumping power is required for the atomization of sunflower oil compared to that of its ester in case of hollow cone atomizer. This is attributed to the smaller size of the air core formed due to higher viscosity of pure sunflower oil. The lower viscosity of bio-diesel causes a higher spray cone angle compared to that with pure vegetable oil for both the atomizers. In case of hollow cone pressure atomizer, the spray cone angle increases with the increase in liquid flow rate, while in the twin fluid atomizer an increase in air pressure increases the cone angle. (C) 2013 Elsevier Ltd. All rights reserved.