The study on the preparation of mono-dispersed liquid droplet was performed using ultrasonic as one of the steps of producing the perfectly mono-dispersed particle. The ultrasonic system was a 33.6kHz ultrasonic generator, an amplifier, an electric distortion element, and a bolted Langevin type horn installed in a cell. For the first time, a liquid pillar under a nozzle of the cell was stably formed. Then, dense and coarse parts were alternatively constructed by oscillating the wavelength of the ultrasonic. The diameter of those liquid droplets were found to be perfectly uniform, therefore, it was measured by "multiplex photography" using a synchronized stroboscope with the ultrasonic. From the results, more than 30,000 of perfectly uniform liquid droplets in 1 second were observed by this method.