Zirconium diboride (ZrB²) is an ultrahigh-temperature ceramic (UHTC) with excellent thermal and mechanical properties. To fabricate a complexly structured UHTC sintered body with high density a high-solid-loading slurry with low viscosity is required to fill the mold well. Extensive studies have been carried out to improve the dispersion properties, including those on solvents and surfactants. In this study, a surface treatment using polyethylenimine (PEI) was investigated. This reagent was used to provide ZrB² particles with desirable properties, such as electrostatic repulsive force and steric hindrance, by the functionalization of amine groups. Additionally, surface treatments with different concentrations of PEI were performed. The appropriate content for a high dispersity was determined. The surface functional groups of the PEI-treated ZrB² were observed by Fourier-transform infrared spectroscopy. The surface charge and enhancement in dispersion were evaluated by zeta potential and particle size distribution analysis, respectively. The rheological properties of the ZrB² slurries with various solid loadings were analyzed by viscosity measurements. In conclusion, ZrB² slurries with solid loadings up to 50 vol% could be prepared through the PEI treatments.