Fumed silica was surface treated by hexyl (C6), octyl (C8), decyl (C10), hexadecyl (C16), and octadecyl (C18) trimethoxysilanes, and the products were evaluated from various aspects. The amounts of alkyl group introduced on silica surface were all within the range from 22 to 25 mmol/100 g. Such fumed silicas were suspended in an epoxy resin, bisphenol type A, and their rheological properties were measured. The rheological behavior of the silica suspensions was investigated as to steady-state viscosity and dynamic moduli in order to examine the effect of alkyl chain length on interactions of hydrophobic groups on the silica surface. The stronger shear thinning behavior was observed in the surface-modified fumed silica with longer alkyl groups. However, the suspensions of the nontreated fumed silica and the fumed silicas treated by C6 silane showed Newtonian behavior even at the silica concentration of 7 wt%. The dynamic moduli G ' of the silica suspensions increased with increasing alkyl chain length. The interaction of hydrophobic groups on the silica surface is mainly attributed to the rheological behavior of fumed silica suspensions in the epoxy resin.