Impedance spectroscopy has been shown to be a powerful tool to investigate the dielectric characteristics of powders suspended in suitable liquids. The electrical and dielectrical contributions of different components of the slurry can be extracted from the impedance spectra through measurement of frequency-dependent relaxations. However, for ferroelectric powders that possess innate surface conductivity, such as BaTiO3, nanoparticles have sufficient conductivity to exclude low-frequency fields that preclude impedance characterization of the particle core. In this work, the slurry technique is shown to be effective for dielectric characterization of not only micrometer-sized particles through equivalent circuit modeling but also applicable to nanometer size dielectric particles upon remediating the conductive surface defect. Application of a self-assembled monolayer (SAM) onto the nanoparticle as a surface passivation layer reduces the surface conductivity, stabilizes the nanoparticles to dissolution, and allows a reproducible measurement and modeling of the nanoparticle dielectric characteristics including nanoparticle permittivity. The dielectric permittivity of surface passivated, ~40 nm diameter barium titanate particles was measured to be εr ~ 135.