In this paper, a non-Newtonian fluid was fabricated dispersing nano-sized silica particles in a polyethylene glycol medium. The rheology of the suspension was investigated in a stress-controlled rheometer under increasing shear rate. Based on the rheological measurements, the suspension exhibited shear thickening behavior which gives a drastic viscosity grow with the increase in the shear rate. In order to investigate the role of the micro-sized additive particles on the rheology of silica based suspension, silicon nitride particles were included in the suspension with three different concentrations. The results were discussed in terms of important parameters for the shear thickening mechanism such as critical shear rate, peak viscosity, thickening ratio and initial viscosity. According to the results, shear thickening behavior can be controlled altering the amount of silicon nitride particles in the suspension.