The nano air-bubbles mixed into gas oil are used for the energy saving and environmental load reduction of diesel engine. The nano air-bubbles were mixed into the fuel line continually by a super miniature ejector-type mixer (outer diameter: 20 mm, length: 34 mm) which can make nano and micro sizes. After the micro air-bubbles were separated with the nano air-bubbles in a mixing tank, diesel engine performance test with a common-rail injection system was experimented. The results showed that 3% reduction in break means specific fuel consumption, 1% rise in charging efficiency and a slight reduction in the density of exhaust smoke etc. These were caused by mixing the nano air-bubbles into gas oil. It is confirmed that the nano air-bubbles had advanced and activated combustion by a physical and chemical action. (C) 2013 Elsevier Ltd. All rights reserved.