An experimental study was conducted to study the effect of ultrasonic vibrations on frost suppression on a cold flat surface in atmospheric air flow. A microscopic visualization on the liquid droplet solidification and frost formation process with and without the effect of 20-kHz ultrasound was made. The water droplets solidification and frost crystal growth on the solidified water droplet surface were comparatively observed. Meanwhile, the frost thickness variations were comparatively analyzed. It was found that almost no difference can be seen between the water droplet solidification onset time with the effect of ultrasound and that without the effect of ultrasound. However, after the water droplet solidifies, almost no growth of the frost crystal can be observed on the solidified water droplets surface with the effect of ultrasound. In addition, during the whole frost formation progress, very small growth of the frost layer can be seen on the flat surface with the effect of ultrasound. The experimental results showed that the frost growth process on the cold flat surface can be significantly restrained due to the effect of ultrasound.