Vibration and noise problems due to fluid flow occur in many industrial plants. These flow-related phenomena are known as "Flow Induced Vibration" (FIV). One of the usual mechanisms of Flow-induced vibration is shedding of vertices behind the bluff bodies. This kind of flow induced vibration is known as "Vortex induced vibration"(VIV). VIV has important effect on increase the drag coefficient of bluff bodies. In this work, the flow induced vibration of tethered sphere was investigated. Experimental study of this work was carried out at towing tank of subsea R D center of Isfahan University of technology. In experimental study effect of relative submergence of sphere and change of natural frequency of tethered sphere and variation of Reynolds number on in-line and transverse force exerted by the flow was measured. Changing the natural frequency of tethered sphere was carried out by changing the length of tether. Effect of changing the natural frequency was investigated by using three various length of tether. By changing the natural frequency of tethered sphere kinematic of movement of sphere will be change and in-line and transverse force exerted by the flow will be change. Experimental and numerical study was carried out in the range of Reynolds number. In this range of Reynolds number, the wake of sphere is fully turbulent and boundary layer on sphere experience transient from laminar to turbulent. Fluid-Structure interaction (FSI) simulations are carried out to investigate vortex-induced vibration of elastically tethered sphere by coupling most famous commercial Computational Fluid mechanics (CFD) codes ANSYS CFX and finite element code ANSYS Structural. Turbulent flow around sphere was simulated using Shear Stress Traort Scale Adaptive Simulation model (SST-SAS) that have suitable accuracy for prediction of separation flow and low computational cost compare with Large Eddy Simulation (LES) model. In numerical study in-line and transverse force exerted due to fluid flow calculated and compare with experimental result. In addition to variation of frequency of forces with Reynolds number was compared. Kinematic of movement of center of sphere was investigated and displacements of center of sphere in both in-line and transverse direction of flow were calculated. Wake structure of oscillating sphere was simulated and compared with non-oscillating sphere. Two distinct mode of vortex induced vibration of sphere was observed that separated with non-periodic transitional regime. In transitional regime between two vibrating modes trajectory of movement of center of sphere becomes chaotic and amplitude of oscillation of sphere increase. In first mode of vibration of sphere displacement of center of sphere in both direction are periodic and trajectory of sphere is elliptic. With increase the Reynolds number displacement of sphere in in-line direction will be increase. The frequencies of exerted forces in both directions are near the natural frequency of tethered sphere. In transition regime of two mode of vibration the frequencies of exerted forces in both directions decrease whereas frequency of oscillation of sphere remains near natural frequency of sphere. Key words: Fluid structure interaction- free surface flow- Turbulence- Flow induced vibration