In the metallurgical industry in China, the manual testing and controlling method are mainly adopted to test and control the steel bars diameter in the manufacturer. They were measured under the cold condition by workers with calipers after steel bar rolling offline, but this method is a kind of random test, which is low in accuracy, slowfooted and has tremendously labor intensive, and it is not able to get a realtime inspection. The rolled products specification and dimension are not able to be controlled immediately, and the rolling mills roller is not able to be changed and adjusted in time. Once the defective items are found, a large quantity of rolled products has exceeded the control area already, which has strong impact on the qualification rate and improving of the efficiency. A method and online measurement system were proposed for the capacitanceinduction type of steel bar. In order to improve the steel bars qualified rate, to make the structure simple and to reduce the cost of equipment maintenance, the highprecision measurement of the steel bars diameter online can be implemented, through test experiment of capacitance in the measurement.