Abstract: Based on the investigation of the existing problems in ultrasonic inspection of large axles, an automated ultrasonic system for on-line inspection of large axles is presented in the paper. The surface of the axle was taken as the benchmark for locating the probe so that the ultrasonic signal could automatically penetrate through the centre of the axel. A magnetic force was used for conducting and attracting the probe to the surface of the axle so that the tracing fixture was small, fast and convenient for installation. The ultrasonic board is based on high speed A/D and PCI bus for high speed of data sampling and transmitting. The system software has all the functions of conventional ultrasonic inspection instruments and advantages of virtual instruments, digital signal process and huge data storage, which significantly improve the quality and efficiency of the ultrasonic inspection.