The gas/liquid coaxial swirling nozzle has the benefits of pressure-swirl and air assisting atomization nozzles. The gas/liquid coaxial swirling nozzle has been widely used in the industries. Due to the procedure complexity for gas-liquid coaxial type of oil injector，by far the research method is mainly experimental. The atomization characteristics of a gas/liquid coaxial swirling nozzle in pressurized space were investigated experimentally. The atomization characteristics were studied by phase Doppler particle analyzer(PDPA)in a line with 1.4 mm diameter in the section located at 70 mm distance to the exit orifice downstream. The droplets radial distribution characteristics and the average diameter of Sauter mean diameter(SMD)affected by oil flux，ambient pressure and atomization air pressure were studied. The results showed that the SMD of the oil droplets was distributed in a saddle shape

the average particle diameters in the center of the atomization torch were smaller and those in the main flow area of the atomization torch were bigger. When atomization air pressure and ambient pressure were fixed，the droplets SMD in the center of the atomization torch decreased and the droplets SMD in the main flow area increased firstly and decreased afterwards as oil flux increased. With increasing oil flux the total SMD of the section increased firstly and decreased afterwards and oil flux inflexion value was 140 ml·s-1. When atomization air pressure and oil flux were fixed，the droplets SMD in the center of the atomization torch decreased and the droplets SMD in the main flow area increased as ambient pressure increased. The total SMD of the section increased as ambient pressure increased. When ambient pressure and oil flux were fixed，the droplets SMD in the center of the atomization torch increased and the droplets SMD in the main flow area decreased as atomization pressure increased. The total SMD of the section decreased as atomization pressure increased. The droplets in the main flow area of the atomization torch，rather than the droplets in the center of the atomization torch dominated the total SMD of the section.