The effect of poly(ethylene glycol) (PEG) on the nonlinear viscoelasticity of Laponite suspensions containing NaCl was investigated with large amplitude oscillatory shear rheology. The molecular weight (M-w) of PEG was 4k, 10k and 35k, and the concentration of PEG was varied from 0.063 wt% to 2.4 wt%. The dynamic strain sweep showed that the nonlinearity appeared at gamma(0) > 30% with a stress overshoot at gamma(0) = 50-70%. The intensity ratio I-3/1 from Fourier-transform increased with gamma(0) when entering the nonlinear regime and leveled off at gamma(0) >= 100% with higher slope and constant value for the PEG of higher M-w or lower concentration. I-3/1 revealed the structure difference in the suspensions induced by adsorbing PEG in the nonlinear regime. The minimum- and large-strain rate viscosities eta(M) and eta(L) from the Lissajous curve were found to be sensitive to the nonlinear viscoelasticity and the peak of eta(M) and eta(L) appeared at lower gamma(0) with higher maximum following the same dependency as I-3/1 on PEG M-w and concentration. The overall nonlinearity parameters N-E and N-V were proposed in this paper and demonstrated to reflect the difference in the Laponite suspensions with PEG more clearly and more effectively. (C) 2011 Elsevier Ltd. All rights reserved.