Wet-type jet milling (WJM) process was utilized to delaminate two kinds of two-dimensional (2D) nanofillers: hexagonal boron nitride (hBN) and graphite. Poly(dimethylsiloxane)s (PDMS) with different dynamic viscosities from 1 to 1,000 mm2/s were employed as process media in order to study the effect of medium viscosity on delamination behavior. Change in particle shape was followed by evaluating both particle width and thickness from image analysis of SEM images. Sedimentation behavior of the slurries was also studied. It was found delamination of 2D nanofillers was induced by WJM treatment in the low viscosity PDMS. On the contrary, the treatment in high viscosity PDMS was ineffective in delamination of 2D nanofillers. Significant increase of equilibrium sedimentation height was observed for hBN slurry treated in low viscosity PDMS. It is considered that both change in primary particle size/shape and aggregation structure contributes to the increase. In the case of graphite, increase of equilibrium sedimentation height was observed even in higher viscosity PDMS, suggesting the importance of aggregation structure.