In this work, La0.95Ca0.05F2.95 solid electrolyte was prepared. The rare earth La sensor was made by using polycrystalline La0.95Ca0.05F2.95 as solid electrolyte, pure La as reference electrode and Mo wire, Mo-ZrO2, as lead wires. The cell can be expressed as : MoLa-(s) or La-(I)La0.95Ca0.05F2.95[La](Fe)Mo-ZrO2 Ceramal. The activity of dissolved La in carbon saturated iron liquid solution (1300 degrees C) with different La concentrations was determined. The results were in agreement with those of oxygen concentration cell e.m.f. determined by oxygen probe. The activity change of La with temperature in the solidification process (from 1350 degrees C) was also determined. The result shows regularity. In the course of the rising and falling temperature, the La probe can be continuously used some times.