We present a lattice QCD study of the phase shift of I = 2 pi pi scattering on the basis of two different approaches: the standard finite volume approach by Luscher and the recently introduced HAL QCD potential method. Quenched QCD simulations are performed on lattices with extents N-s = 16, 24, 32, 48 and N-t = 128 as well as lattice spacing a similar to 0.115 fm and a pion mass of m(pi) similar to 940 MeV. The phase shift and the scattering length are calculated in these two methods. In the potential method, the error is dominated by the systematic uncertainty associated with the violation of rotational symmetry due to finite lattice spacing. In Luscher's approach, such systematic uncertainty is difficult to be evaluated and thus is not included in this work. A systematic uncertainty attributed to the quenched approximation, however, is not evaluated in both methods. In case of the potential method, the phase shift can be calculated for arbitrary energies below the inelastic threshold. The energy dependence of the phase shift is also obtained from Luscher's method using different volumes and/or nonrest-frame extension of it. The results are found to agree well with the potential method.