Abstract The effect of reducing cavitation damage in high-speed flow structure not only relate with entrained air concentration, but also relate with air bubble density has been discovered in recent research. The air bubbles in aerated flow with diameter below then 0.2mm or 0.5mm plays important role in reducing cavitation damage. May be very little air concentration can protect construction free from cavitation erosion. Thereby, measuring bubble size must be emphasized in study of reducing cavitation damage. Measuring bubble size can use probe of needle-type and image usually. By analyses and application experience of velocimeter of air concentration probe of needle-type, suggesting of the median size of bubble d 50 adopt measuring data by probe directly, and calculate space distance and density of bubble. Although, the d 50 and space distance would be big then real size, and density small then real value. The measuring results have importance value for studying mechanism and establishing a new criterion on entrained air to reduce cavitation damage.