Agglomeration of inclusions in liquid steel causes not only the enhancement of inclusion removal by flotation but also the increase in the number of large inclusions in final products. To clarify agglomeration behavior theoretically, a lot of studies have been made until now. However, the behavior is not clearly understood yet. In this study, a new particle-size-grouping (PSG) method has been established, which enables a simple calculation of the agglomeration by a small number of size groups with complete conservation in total particle volume. This method has been verified by the comparison with the exact solution of a revised population-balance equation. An experimental study of the agglomeration of polyvinyl-toluene latex (PVTL) in a stirred electrolyte solution has been made in an agitated vessel under a rapid agglomeration condition. An effective Hamaker constant of PVTL in water, A₁₃₁, has been obtained by adjusting the measured agglomeration curve with the curve calculated by the PSG method. Good agreement has been obtained between observed and calculated agglomeration curves for A₁₃₁=0.8×10^-20J under a wide range of initial particle concentrations and agitation speeds. Numerical simulations of the fluid flow and particle transport in the vessel have been made to confirm the applicability of the PSG method. Computed agglomeration curves agree well with the theoretical curve if the energy dissipation rate averaged with the residence time of liquid in computational cells is used to calculate the dimensionless agglomeration time.