A simulation model to predict the dynamic behavior of large aggregates in flows was developed, in which the effective particle surface exposed to the flow was evaluated and the reduction of fluid velocity due to neighboring particles was taken into account. This model was applied to predict the dynamic shape factors of regular shaped aggregates settling in a quiescent fluid and the simulated results were compared with the experiments reported elsewhere. It is found that the hydrodynamic drag force exerted on aggregates is simulated quantitatively by the present model. This indicates that the proposed simulation model is applicable to predict the complicated behavior of large aggregates in flows, such as their deformation and breakup.