The rheological behavior of epoxy resin filled with glass hollow microspheres was studied using three types of microspheres with different particle size distributions. The shear-rate dependence on viscosity, as well as relative apparent viscosity, of the suspensions was investigated at different microsphere filler loadings. The change in relative viscosity with different types of microsphere loads was also investigated using the established semi empirical model to estimate the critical filler load. It was found that the critical filler load for all suspensions tested depends mostly on the filler size distribution, neither on the average filler size nor on the specific surface area of the filler.