Effects of microbubble addition in the sonochemical reaction system on the metal nano particles formation were investigated. The reactant was an aqueous solution of a metal salt and sodium dodecyl sulfate (SDS); the latter was added as a reduction reagent for the metal ions and as the dispersant. With the radiation of ultrasound, the cavitation phenomena will occur in the aqueous phase, which leads the reaction system under high pressure and high temperature conditions. As a result, some radicals will be formed in the system that will be involved in the reduction of metal ions. Since the cavitation is quick cycles of formation and collapse of microbubbles in the aqueous phase, it is expected that the addition of microbubbles into the reaction system beforehand will enhance the reduction rates due to the increase in the active reaction filed. In addition, the addition of microbubbles with a controlled size could result in the formation of metal nano particles with a controlled size. To examine the above concept, experimental studies have been conducted in this study. The microbubbles were obtained by a method based on the “bubble point principle”. A porous polymeric membrane was mounted in a high-pressure membrane cell (Fig. 1, 1); one side of the membrane was contacted with a reactant solution (mixture of metal ions and a surfactant, SDS), and the other side of the membrane is contacted with a gas phase (normally argon). When the gas phase pressure was increased above the “bubble point” pressure, the gas will be pushed out into the aqueous phase through the pores in the membrane, and the gas phase will form bubbles on the surface of the membrane. The size distribution of the microbubbles formed with the present method was dependent on the membrane pore size, membrane material, and the concentration of the surfactant. Microbubbles with a relatively uniform size distribution at 20 ~ 40 μm were obtained when a polycarbonate membrane with the pore diameter of 0.025 μm was used. The formation rate of the microbubbles could be controlled by changing the gas phase pressure. The bubble formation cell was connected to the sonochemical reaction system with circulation. The reactant solution containing the microbubbles formed in the membrane cell was then introduced to the sonochemical reaction system. The reaction tube is located on an ultrasonic irradiation apparatus, of which the frequency is 170 kHz and the power is 470 W. When silver nitrate solution (10 mM) was used as a reactant with SDS (32 mM) was used as a reactant, an acceleration effect of the reduction reaction of silver ions was confirmed by the UV absorption spectroscopy of the reactant; the absorption at 400 nm, which can be attributed to plasmon absorption of silver particles, was increased with the addition of microbubbles. This result indicates that, the reduction rate of silver ions increased with an increase in the amount of the microbubbles added. The formation of silver nano particles were confirmed by the TEM observations. The mean particle size of silver with the microbubble addition was about 4.6 nm (SD = 1.1 nm), which is in the same range without microbubbles.