In the wire EDM using a thin wire electrode, the reduction of wire vibration is important to achieve a high-precision machining, since large amplitude of wire vibration leads to large kerf width, low shape accuracy, rough machined surface and high risk of wire breakage. In this study, the movements of wire electrode are directly observed by using a high-speed video camera in order to clarify the wire vibration in fine wire EDM for thin workpiece. The results shows that the wire vibration amplitude during the process increases with wire running speed and flow rate of jet flushing but decreases with wire tension. Also, the wire behavior during process is almost 1st order string vibration. Furthermore, the wire vibration frequency increases with not only the wire tension but also the discharge pulse frequency.