Nanocrystalline LaCo_0.3Fe_0.7O₃ was prepared by a citrate method. A resistive-type humidity sensor built on a ceramic substrate with Ag–Pd interdigitated electrodes was fabricated. The humidity sensing mechanism of the ceramic materials was assessed by analyses of DC and AC test results. Electrons and ions contributing to the conduction were confirmed by DC analysis. Electrons, H⁺, and K⁺ ions were each found to dominate at different humidity ranges for the humidity sensor based on LaCo_0.3Fe_0.7O₃ materials. The dielectric loss property was studied in detail and the humidity sensing mechanism was discussed. The results indicated that not only proton or ion but also molecular polarization contributes to the conduction in 11–95% RH.