The effect of moisture on the Li ion conduction of Li7La3Zr2O12 (LLZ) ceramic and the electrochemical properties of LLZ-based solid state battery was investigated using LiFePO4 (LFP) film as a cathode. LFP film was fabricated by aerosol deposition method. The secondary phase was formed in LLZ ceramic, when the specimen was exposed in humid air. With the formation of the secondary phase, the severe degradation of Li ion conduction was observed at the grain boundary. In addition, the charge and discharge capacities of LLZ-based solid state cell were quite low at room temperature, although, at 140 °C, the cell showed the high capacity. Therefore, the protection of LLZ ceramic from moisture might be required to gain the high capacity of LLZ-based solid state cell at room temperature.