In order to investigate the nonlinear stress relaxation behavior of viscoelastic polymer liquids at large shear deformations, the relaxation modulus G(t, r) of concentrated aqueous poly(ethylene oxide) (PEO) solutions has been measured with a Rheometrics Fluids Spectrometer (RFS II) over a wide range of shear strain magnitudes. The strain dependence of G(t, r) at various molecular weights and concentrations was reported in detail, and the result was interpreted using the Doi-Edwards theory. In addition, the time-strain separability (or factorability) of the nonlinear relaxation behavior was examined by superposing the G(t, r) curve on the linear relaxation modulus G(t) surve through a simple vertical shift. The experimentally determined damping function h(r) was compared with the results calculated from some empirical equations proposed by earlier researchers, and finally the effects of molecular weight and concentration on h(r) were discussed. Main results obtained from this study can be summarized as follows : (1) The G(t, r) curve at small range of strain magnitudes shows a linear relaxation behavior, which is independent of the deformation magnitude. As the strain magnitude is increased, however, the G(t, r) curve deviates from the linear relaxation behavior and falls successively below the G(t) curve. (2) When sufficiently large strain magnitude is imposed on the highly concentrated solutions polymer of high molecular weight, the G(t, r) curve shows a more complex shape having the two inflection points. This behavior is due to the occurrence of a new relaxation process, and can be accounted for by the retraction process of primitive polymer chains in the tube, as predicted by the Doi-Edwards theory. (3) The G(t, r) is separable (or factorable) into a time-dependent function G(t) and a strain-dependent function h(r) when the time region is longer than the experimentally determined material time constant ${lambda}_{k}$. The ${lambda}_{k}$ increases with increasing molecular weight and concentration. (4) The empiricisms proposed by Wagner and Osaki cannot provide an adequate description to predict the nonlinear relaxation behavior. In contrast, both the Zapas and Soskey-Winter relationships are in excellent agreement with the experimentally determined h(r). The h(r) shows a stronger dependence on the strain magnitude with increasing molecular weight and concentration.