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Fluoride ion sensor electrodes that use polycrystalline LaF3 prepared by low‐temperature sintering can reduce sensor size and fabrication costs; however, decreasing the sintering temperature deteriorates sensor performance. Further, it is unclear why the performance of polycrystalline LaF3 prepared at low temperatures is inferior to that of the single‐crystal LaF3. Thus, in this study, we investigated factors that deteriorate sensor response under low sintering temperatures, such as solution penetration, fluorine conductivity, and ion exchange. To this end, LaF3 was prepared by changing the sintering temperature, and the electrical characteristics and sensor performance were evaluated and compared with those of the single‐crystal LaF3. The sensor performances of LaF3 sintered at 900 °C (−9.3 mV/dec) and 1000 °C (−5.8 mV/dec) were inferior to that of a single‐crystal LaF3 (−49.9 mV/dec). LaF3 prepared by sintering over 800 °C achieved fluorine removal and showed lower ion conductivity and ion exchange compared to that of the single‐crystal LaF3. LaF3 sintered at 700 °C did not show a sensor response because of solution penetration into the through‐pore. Our results confirmed that solution penetration, ion conductivity, and ion exchange influence the sensor performance of LaF3. Finally, we discussed approaches to achieve high sensor performance with LaF3 prepared by low‐temperature sintering. The proposed approaches are expected to contribute to expanding the utility of fluoride ion sensor electrodes with LaF3. [ABSTRACT FROM AUTHOR]