Recently, various types of flexible devices such as flexible displays, batteries, e-skins and solar cell panels have been reported. Most of the researches focus on the development of high performance flexible device. However, to realize these flexible devices, the long-term reliability should be guaranteed during the repeated deformations of flexible devices because the direct mechanical stress would be applied on the electronic devices unlike the rigid Si-based devices. Among various materials consisting electronics devices, metal electrode is one of the weakest parts against mechanical deformation because the mechanical and electrical properties of metal films degrade gradually due to fatigue damage during repeated deformations. This article reviews the researches of fatigue behavior of thin metal film, and introduces the methods to enhance the reliability of metal electrode for flexible device.