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A silver nanocolloidal solution was prepared by a cavitation bubble plasma with a pair of silver rod electrodes in a sodium hydroxide water solution with the initial pH = 11.0. The cavitation bubble plasma was generated by the bipolar high voltage pulses of the repetition rate 200 kHz and the pulse width 0.8 μs in the sodium hydroxide water solution in which many cavitation bubbles produced by a high-speed rotor were feeding. The UV–Vis absorption spectra of the silver nanocolloidal solution exhibited a characteristic peak around 400 nm in wavelength. The silver nanocolloidal solution of 250 mL including silver nanoparticles of median diameter 1.3 nm and concentration 4 ppm was prepared at the processing time 1 min. The particle size in the solution increased with increasing the processing time and grew to 14.6 nm in median diameter at the processing time 20 min. The concentration of particles also increased with increasing the processing time. A TEM image of silver nanoparticles revealed that the particles were spherical crystalline.