Transparent electrodes are widely applied in optoelectronic devices such as flat panel displays, photovoltaics, lightings, thin film transistors, etc. As the long-term usage of the devices is very important, the transparent electrodes should have good electrical stability. Al-doped ZnO (AZO) transparent electrode has been considered as the best replacement of the mostly used transparent electrode, Sn-doped In 2 O 3 (ITO), however, the electrical stability of AZO thin films is yet to be satisfactory. To improve it, this research investigated the influence of carrier concentration on the electrical stability. AZO thin films were prepared on glass substrates by RF magnetron sputtering at 100W of RF power with deposition duration of 60 minutes with a Zn metal target and 3 Al chips. Argon and oxygen gases were applied both at 30 sccm. Then thin films were annealed for 30 minutes in a hydrogen atmosphere at 400 to 550°C. After damp heat test with 85% of relative humidity at 85°C for 30 days, higher-temperature-annealed AZO thin films obtained better stability. Decrease of carrier concentration was more influential to the electrical stability than the decrease of mobility. Top oxide layer, which seemed to be spontaneously formed at higher temperature was effective to keep the carrier concentration.