In this work, the effects of the concentration of graphene oxide (GO) and poly(ethylene glycol) (PEG) on the colloidal and rheological behavior of aqueous GO dispersions were investigated. The concentration of GO and PEG was varied from 0 to 20mg/mL, respectively. Atomic force microscopy (AFM) images and zeta-potential measurements demonstrated that PEG chains were adsorbed on the colloidal GO sheets. The red-shift of D and G bands in the Raman spectra of GO/PEG compared with pure GO suggested that there was hydrogen bond interaction between GO and PEG. Steady state shear results indicated that GO aqueous dispersions changed from Newton fluids to pseudoplastic fluids as the concentration of GO increased. The critical concentration of isotropic-nematic phase transition of aqueous GO dispersions was about 6mg/mL. Significantly, the aqueous GO/PEG dispersions presented a lowest viscosity and minimum linear storage modulus linear G' at the saturation adsorbing concentration cp,s, which were determined by steady state shear and small amplitude oscillatory frequency sweep, respectively. Therefore, the PEG concentration had a significant effect on the linear and non-linear rheological behavior of GO/PEG dispersions. In addition, the concentrated aqueous GO dispersions showed typical yield flow behavior and the yield stress σy firstly decreased and then increased with the increasing of GO concentration. These results could contribute to understand the interplay between microstructure and mechanical behavior of GO/polymer in aqueous dispersions, and also provide some guidance for the processing of GO-based polymer nanocomposites.