This paper is concerned with methods of generation, control and measurement of micro-air-bubbles in water. A new device for the generation of micro-air-bubbles is developed on the basis of dissolution and separation processes of air in water. In the dissolution process, air is dissolved in water in such an efficient way that compressed air is ejected into water in the form of numerous small bubbles from a rotating circular porous plate and that the mixture of water and generated air bubbles is stirred in the following mixing box where air bubbles dissolve into the surrounding water. In the succeeding separation process, the air-dissolved water is discharged through a pressure reduction valve, after which the dissolved air becomes supersaturated and then the air is separated in the form of micro-air-bubbles. The diameter and the number density of separated micro-air-bubbles are controlled by adjusting both the rotational frequency of the porous plate and the flow rate of air ejected into water so as to change the amount of air dissolution in water. Experimental results show that the mean diameter of micro-air-bubbles is controlled between 10 and 20 mum, and the number density between 1.3 x 10(2) and 6.6 x 10(2) number/mm(3). (C) 2003 Elsevier Ltd. All rights reserved.