A study was conducted on the effect of particle aggregation on the rheological characteristics of an electrorheological (ER) fluid of the BaTiO3 type as a colloidal ER fluid. Increasing viscosity by applying an electric field was measured using a rotating viscometer. The experimental data was compared with the theoretical analysis taking into account either uniform particle dispersion or particle aggregation. The effect of aggregation on the rheology of the ER fluid was clarified and the increasing viscosity was quantitatively determined by a formula based on the aggregation model.