The objective of this study is to investigate and characterize the mechanism of the island wake behind an island called Liu Chiu Yu off the southwestern Taiwan coast based on the in-situ data of Sb-ADCP, CTD and satellite images. The findings suggest that a counter-clockwise eddy and a clockwise eddy both are with 0.01 S vortice appears in the wake of Liu Chiu Yu when the background flows are toward the northwest. The system of two eddies with opposite rotation and a central return flow develops an unsteady eddy shedding. On the other hand, when the background flows are toward the southeast, island wake generated in the lee of Liu Chiu Yu is attached system of two eddies with opposite rotation and a central return flow. The Sb-ADCP data shows that the flow pattern in the northeastern coast of Liu Chiu Yu is mainly semidiurnal. The major axis of the ellipse of the semidiurnal current is parallel to the orientation of the coast line (northeast to southwest) and the shape of the ellipse is quite long and narrow. The amplitude of the semidiurnal current is approximately two times that of the diurnal current. Generally, the currents are stronger and the occurring probability of the island wake is higher during spring tide. The CTD data shows that the eddy center appears to be divergent and upwelling occurs in the areas under the influence of island wakes. The upwelling pumps deep seawater to the surface and results in low temperature, high salinity, high oxygen concentration and low chlorophyll concentration. On the other hand, in the eddy edges, downwelling occur causing high temperature, low salinity, low oxygen concentration and high chlorophyll concentration. Strong shear was formed at the depth of 60m inside the island wake which generates thermocline so that the mixing phenomenon is quite obvious there. Moreover, in the regions without the influence of island wakes, the stratifying effect is clear and the horizontal variation of temperature, salinity and oxygen concentration is small. Therefore, neither upwelling nor downwelling occurs there. Furthermore, along the edge between blocking and free-stream areas, the shear stress increases and the mixing phenomenon arises to a certain degree. The satellite images show that an island wake appears in the southeastern Liu Chiu Yu during spring tide. The island wake develops a phenomenon called von Karman vortex street. At the same time, a counter-clockwise eddy with heavy suspensions appears in the northern Liu Chiu Yu. The radius of the eddy is around 4 Km. The area of the lowest chlorophyll concentration is located at the center of the eddy. By analyzing all these data, it is concluded that the island wake in Liu Chiu Yu usually appears during spring tide.