When a liquid is irradiated with high intensity ultrasound, unusual and unique reaction-sites are formed. They are attributed to extremely transient and small cavitation bubbles with high temperatures and high pressures. The chemical reactions and their reaction pathways during cavitation are often different from those of conventional thermal-and photo-chemical reactions. We have been investigating the reactivities of active radicals formed from cavitation bubbles and have reported that reduction by radicals could be applied for preparing various noble metal nanoparticles. In this paper, we describe the mechanism of the sonochemical reactions occuring in the formation of metal nanoparticles and theproperties of the synthesized particles.