Aggregate structures of a colloidal dispersion composed of monodisperse and polydisperse ferromagnetic particles were simulated by the cluster-moving Monte Carlo method. The internal structure of aggregates was analyzed by means of the radial distribution function, and the cluster size distributions and angular distribution functions were investigated. The results obtained are summarized as follows. In the monodispersive system, chain-like clusters are formed and become longer as the magnetic particle-particle interactions increase. In the polydispersive system, chain-like, ring-like and lump-like clusters are formed. As the magnetic interactions increase further, these clusters gather and form larger clusters which have a network structure. This is mainly induced by the influence of larger particles which have stronger magnetic interactions.