In this paper, 3D investigation has been employed to study the wake instability of viscoelasic fluid flow behind unconfined sphere. For estimating the proper properties of the viscoelastic fluid in this study a non-linear Giesekus model is used as the constitutive equation of viscoelastic fluid. Numerical computations are carried out by solving the governing and the onstitutive equations of the viscoelasic fluid flow using the finite volume technique and OpenFOAM which is an open source code is used as the CFD solver. At first velocity field and flow streamlines of Newtonian fluid around the sphere for various Reynolds numbers have been plotted and by plotting the velocity magnitude and pressure at a point behind the sphere versus time, the value of Recr in which the flow become unstable has been reported. Furthermore, for validating the present numerical code, variation of drag coefficient around the sphere versus Reynolds number has been compared with previous investigations. In the following, the effect of Reynolds and Wisenberg number on fluid flow and instability of wake formation behind a sphere have been investigated at high values of Reynolds number for the first time. Results show that at high values of Reynolds number the effect of Wisenberg number has less effect in contrast with Reynolds number on flow instability behind the sphere.