This paper describes a fuel atomizer developed for S.I. engines based on ultrasonic vibrations. As the spray is characterized by fine droplet size and low penetration, it facilitates fuel movement and the formation of a homogeneous mixture. The spray behavior of this atomizer is easily influenced by ambient air motion. Therefore, the spray is most effectively delivered to the cylinders by precise injection timing. The ultrasonic atomizer disperses a fine spray over a wide flow rate range. A single cylinder engine fitted with the atomizer showed advantages in combustion speed and transient response performance.When used in a commercial SPI engine, the ultrasonic atomizer maintained constant A/F distribution in all cylinders with no geometrical modification of the intake manifold. In cold weather conditions, the atomizer demonstrated excellent startability. The ultrasonic atomizing system, including a microcomputer unit, effected a noticeable fuel savings advantage in the Japanese 10 mode cycle test on a passenger car. Furthermore, superior acceleration was achieved, particularly in the high speed region.