Effect of ultrasonic treatments (28, 45, 100kHz) on Saccharomyces cerevisiae was investigated with fluorescence stained organelle images using a confocal laser-scanning microscope (CLSM) and with a formation ability of cell colonies. The ultrasonic treatment, especially lower frequency as 28kHz, was effective for sterilizing S. cerevisiae cell. The membrane collapses of cell and organelle caused by ultrasonic treatment were observed by CLSM images.