Abstract: A novel electrochemical oxygen sensor has been developed by using Laβ-Al₂O₃ as solid electrolyte and Cr+Cr₂O₃ as reference electrode. The sensor not only can be used as normal oxygen sensor but also as an ultra-low oxygen sensor. Especially, it is very sensitive to measure ultra-low oxygen in molten metal. For estimating the accuracy of Laβ-Al₂O₃ oxygen sensor, two series of oxygen activities in molten iron at different oxygen contents and different temperature were measured by both Laβ-Al₂O₃ oxygen sensor and ZrO₂ oxygen sensor. The theoretical values of oxygen activities in molten iron (3.30%C, in mass fraction) at 1723K and 1745K were also evaluated for comparing the measuring results of two sensors. At last, the error of measurement for Laβ-Al₂O₃ oxygen sensor was discussed too.