In this study, the rheological behavior of fumed silica nanoparticles suspended in polyethylene glycol (PEG) and those containing multi-walled carbon nanotubes (MWNTs) was measured at steady and oscillatory shear stress using a stress-controlled rheometer. The obtained results showed that the critical viscosity of the concentrated silica suspension, decreased with the addition of MWNTs. Moreover, the onset of shear thickening for the suspension containing MWNTs appeared at higher shear rate than those without MWNTs. Oscillatory shear measurement shows that the storage modulus and loss modulus of silica suspension were higher than the suspension containing MWNTs. The higher complex viscosity of silica suspensions than the suspension containing MWNTs was also evident over the entire range of the frequency studied. The proposed mechanism of nanotube incorporation in dispersion revealed that the observed shear-thickening behavior of a suspension containing MWNTs could be attributed to the increased interaction force between MWNTs and PEG, as a result of increase in the number of hydrogen bonds. This finding suggests that MWNTs could be used as an efficient tool for tuning of the onset of shear thickening and rheological behavior of concentrated silica suspensions.