Linear viscoelastic behavior was examined for suspensions of an ultra-fine silica particle, S7.5, with a radius of 7.5nm dispersed in highly viscous medium consisting of ethylene glycole and glycerin, and was compared with that of previously examined suspensions with radii larger than S7.5. A S7.5, system exhibited remarkable viscoelasticity as well as systems with larger radii, and the zero-shear viscosity, η₀, the high frequency limiting viscosity, η_∞′, and the high frequency limiting modulus, G_∞, could be estimated. However, the dependence of η₀ and η_∞′ on particle concentration for the S7.5 system was completely different from universal one holding in the larger particle systems independently of particle radii. The size of the S7.5 is so small, that the contribution of a special interface layer formed by interacting medium molecules with the surface of the S7.5 to gross rheology becomes important in contradiction to the larger particle systems. The relationship between η₀/η_∞′ and patticle volume fraction, φ, holding in the larger particle systems which are hard-core suspensions was extended to the S7.5 system. Then, the effective volume fraction, φ_eff, and the effective medium viscosity, η_m⁺, for the S7.5 system was estimated by rescaling the system to an equivalent hard core suspension system. The average relaxation time for the system could be successfully regulated by the φ_eff not by φ, however, the G_∞ could be simply governed by the original φ just like found in the hard core suspensions.