Abstract: In this paper, an on-line ultrasonic H 2O 2 concentration meter operating according to the relationship between the concentration, temperature and ultrasonic velocity in H 2O 2 solution is introduced. The ultrasonic velocities in H 2O 2 solution are measured at different concentrations and temperatures.The velocity-concentration coefficients and velocity-temperature coefficients of H 2O 2 solution are calculated. In the on-line ultrasonic H 2O 2 concentration meter, the ultrasonic transducer installed outside the H 2O 2 solution pipeline generates an ultrasonic wave pulse. The ultrasonic wave travels through the pipeline of H 2O 2 solution. The computer transfers the ultrasonic travel time into ultrasonic velocity. The platinum resister is installed inside the H 2O 2 solution pipeline. The resistor variation caused by temperature is sent to the computer through the interface. The computer calculates the ultrasonic velocity and H 2O 2 temperature by means of velocity-concentration coefficients and velocity-temperature coefficients. It displays and prints the value of the instant H 2O 2 solution concentration, and meantime displays the 8 hour curve for H 2O 2 solution concentration variation in the screen. It also can give alarm and output a 4—20 mA analogue signal for automatic control in the production process.