In order to discover the hidden trouble of the 10kV transformer, in this paper, on-line monitoring system based on LabVIEW is developed. The paper briefly introduces the basic concept of the virtual instrument and the frequency-response analysis (FRA), and then it expounds the development process of the hardware and software of this system. The hardware performs the main functions: signal injection, filtering, amplifying, signal conditioning, and storing the result in computer. The software includes four modules: user login, data acquisition, data analysis, data query. Finally, the monitoring system is validated by a corresponding experimental study.