In this paper, we investigated charge stability of electrets made of 10 dielectric materials to examine what material properties are important factor for the stability. Since long lifetime is preferable for electrets energy harvester, charge stability of electret is one of the most important issue in performance optimization. Stability test revealed that water absorption of the dielectrics seems to affect charge stability. Effect of charge stability on output power of energy harvester was also investigated using rotational energy harvester. Using two dielectric materials as electret, output power measured at 0 hour and 44 hours later from charging confirmed that charge stability of electret significantly affect lifetime of energy harvester.