A non-destructive online measurement system called “Bainite Sensor” is developed for the application of monitoring bainite transformation in salt bath adapted heat treatment equipments, nowadays controlled by computer assisted features. The measurement procedure is based upon eddy current harmonic analysis. Electrical and magnetic properties are measured and correlated to the relevant material properties. The Bainite Sensor device, housed temperature resistant and vacuum sealed, can also to be used for online process control. The system was studied carefully during an applied research program. Many experiments controlling the influence of sensitive parameters, such as state of austenitization and bainite transformation conditions of several ball bearing steels have been made and results evaluated. The investigations show that the Bainite Sensor allows the continuous in situ measurement of the bainite transformation. The most important constituent of the microstructure is the proportion of mother-austenite, which is measured continuously. It is possible to reproduce process cycles with a defined fraction of residual austenite as well as finishing the bainite transformation with any content of retained austenite without any delay by a time security factor. The application of the Bainite Sensor makes the bainitic treatment for hardening more reproducible, well directed and more economical. This innovation will improve the industrial competitiveness and foster the development of energy-efficient low-cost quality-assured bainitic treatment routines featuring of by improved properties such as reduced distortion, increased toughness, and sufficient surface hardness.