The principle for selecting the reference electrode of oxygen sensor for steelmaking was explained by the distribution formula of oxygen potential in a ZrO2-based solid electrolyte, and this formula was evaluated under steady-state condition. The Cr/Cr2O3 reference electrode is better than the Mo/MoO2 reference electrode to measure the steel melts with oxygen content lower than 300 ppm at 1600 degrees C. The use of Mo/MoO2 reference electrode can accurately measure the steel melts with oxygen content higher than 300 ppm, but is not suitable for measuring the steel melts with extremely low oxygen content.