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 hot 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. 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 hot also as an ultra-low oxygen sensor. Especially, it is very Precise 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 both 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.