Emulsifying action of high-frequency acoustic waves was investigated on surfactant-free oleic acid/ water mixtures using megasonic irradiation (200 kHz and 1 MHz) in baths manufactured for precision cleaning. While the droplet size distribution was bimodal for single irradiation at 40 kHz or 1 MHz, a unimodal distribution was obtained at 200 kHz irradiation. The whole amount of dispersed oil was evaluated with the total organic carbon measurement. The largest dispersed amount was obtained for the 40 kHz treatment, while the amount was quite small for 200 kHz or 1 MHz treatment. Then, a tandem emulsification method was devised for oleic acid/water emulsion, in which the first irradiation was made at 40 kHz, followed by the second one at 200 kHz over varying exposure times. A primary peak appeared at 100 nm droplet size accompanying with a subpeak at 400 nm in the first dispersion. The latter population decreased with increase in exposure time in the second dispersion until it disappeared. This indicates that the large droplets are disrupted to form or release the smaller ones as a result of sonic acceleration. The averaged droplet size showed a preliminary increase before being stabilized within 2 days in the 40 kHz --> 200 kHz and 40 kHz --> 200 kHz --> 1 MHz sequential processing. Contrary to a low stability for the emulsion obtained at 40 kHz exposure, the turbidity for these emulsions could be observed for 1-2 years. A slight rise in the zeta potential was noticed for the droplet at the early stage. These results demonstrate that sequential processing with megasonic waves has a high potential to improve characteristics of unstable emulsions in surfactant-free condition.