This paper describes an ultrasonic on-line sensor for monitoring molten magnesium (Mg). This sensor provides high spatial resolution measurements in molten Mg in a pulse-echo mode. The developed sensor mainly consists of a conventional piezoelectric ultrasonic transducer and a clad buffer rod as a probe. The martensitic stainless steel (JIS type, SUS410) is chosen as material for the buffer rod so that the rod can sustain an adverse condition in molten Mg. The performance of the sensor, the wave propagation characteristic and the rod sustainability for long time operating time, have been evaluated experimentally. The possible reaction between the developed sensor and liquid Mg has been discussed. Based on such preliminary investigation, an experiment in molten Mg at temperature 973 K has been carried out. Ultrasonic signals from a steel object immersed in molten Mg are clearly observed using the developed sensor at 5 MHz.