In the present studies on the current energy extracting from vortex-induced vibration (VIV), most physical models are simplified as 2D circular cylinder, few of them analyzed the 3D effects of the endplates. In this study, the VIV tests of single-DOF circular cylinder with 6cm diameter are conducted in the circular tunnel with the velocity changing from 0 to 0.75m/s. Five endplates with different shapes and seven rectangular endplates with different lengths are considered. Based on the test results, the optimal shape and optimal size of endplate for the energy extracting are investigated. The main conclusions are listed as follows: The optimal shape is rectangular, and the unfavorable one is no endplate; The amplitude and frequency of the cylinder with rectangular endplate are 1.38 times and 1.25 times of that with no endplate cylinder respectively; The length of endplate on the in line direction (the flow direction) should be controlled in the range of 1.5 to 2 times the diameter of the cylinder to enhance the vibration.