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Atomization characteristics of an ultrasonic fuel injector using a micro nozzle array were improved by change of horn geometries. Micro nozzles whose exit diameter d = 3 µm are mounted on a thin metal film, and the number of micro nozzles was varied from 2.0 × 104 to 1.2 × 105. Gasoline is periodically pushed out from the micro nozzles at frequencies from 62 to 65 kHz. A disk type PZT (Lead zirconium titanate) is used as an ultrasonic oscillator, and the oscillation is amplified by an axi-symmetric step-type horn. The oscillation amplitude is proportional to the area ratio of the horn. The number of micro nozzles increases with the increase in the small end diameter of the horn, while the oscillation amplitude decreases at the same time. In order to increase the fuel flow rate, the horn small end diameter Ds was varied from 10.5 to 25 mm, while the large end diameter is fixed at 30 mm.