Characterizing dispersion quality of polymer nanocomposite (PNC) is as important as dispersing nanosized fillers in the polymer matrix. In this study, to quantify the dispersion quality of PNCs, we used rheological properties, that is, linear viscoelasticities determined by small-amplitude oscillatory shear tests and nonlinear viscoelasticities determined by large-amplitude oscillatory shear tests. Nonlinear viscoelasticities were analyzed with Fourier transform (FT)-rheology. Two different disperse-controlled PNCs were investigated. One is a polypropylene (PP)/clay nanocomposite system compatibilized using maleic anhydride-grafted polypropylene (MAPP), and the other is a PP/silica nanocomposite system containing four different types of silica (two hydrophilic and two hydrophobic silicas). Rheological measurements and transmission electron microscopy (TEM) findings for both PNCs, and X-ray diffraction (XRD) findings for PP/MAPP/clay were used to investigate morphological evolutions. In the case of PP/MAPP/clay nanocomposites, dispersion qualities, as characterized by linear and nonlinear rheological properties, were consistent with each other and well matched TEM and XRD observations. In contrast, in the case of PP/silica nanocomposites, dispersion qualities as characterized by linear and nonlinear rheological properties were inconsistent with each other. In this study, dispersion states of PNCs predicted by nonlinear rheological properties corresponded with TEM observations, whereas linear rheological properties did not. Especially, NLR (nonlinear-linear viscoelastic ratio equivalent to normalized nonlinear viscoelasticity as determined by FT-rheology/normalized linear viscoelasticity) parameter well predicted dispersion degrees of PP/MAPP/clay nanocomposites and PP/silica nanocomposites.