To develop a compact generator of bulk nanobubbles (ultrafine bubbles), ultrasound was irradiated to ultrapure water. Effects of ultrasonic power and frequency on nanobubble concentration and diameter were investigated. Nanobubbles with a diameter of 90-100 nm were generated. Number concentration of nanobubbles increased with time and approached asymptotically to an equilibrium value which was about 1.5 x 10(9) mL(-1). Nanobubble concentration increased with increasing ultrasonic power and decreasing frequency. For comparison, ultrasound was also irradiated to high concentration nanobubble water prepared by pressurized dissolution method. Reduction of nanobubbles was measured and nanobubble concentration decreased to the equilibrium value. Nanobubble concentration decreased with increasing ultrasonic power and frequency. Generation and reduction of nanobubbles by ultrasound was modeled to analyze experimental data. The generation rate coefficient of nanobubbles increased with decreasing frequency because cavitation collapse became stronger. The reduction rate constant of nanobubbles at 488 kHz and 1 MHz in high concentration nanobubble water became much higher than those in ultrapure water due to aggregation and floatation of nanobubbles. The equilibrium number concentration increased with decreasing frequency. The results calculated by our model were in good agreement with the experimental data. The nanobubble generator using ultrasound is compact, simple in operation and produce nanobubbles in a short time. This method is contamination-free because the liquid pump is unnecessary. (C) 2018 Elsevier Ltd. All rights reserved.