Abstract: An on-line measurement system for size concentration by ultrasonic technology is introduced.According to the principle of ultrasonic measurement,the system is made of ultrasonic measurement device,interface unit and computer.The signal of transit time,which is detected by ultrasonic measurement device,is inputted into the computer through the interface unit,and is processed by the LabVIEW application software.At the same time,it makes use of function relationship,which describes the relationship between size concentration and transit time of ultrasonic,gets the real-time size concentration by calculation,and displays it on the screen of the computer.It is proved by comparison with the oven dry method that the error of the size concentration detected by this on-line measurement system using ultrasonic technology is lower than 0.5%.