Aggregate structures of a colloidal dispersion composed of polydisperse ferromagnetic particles were investigated by the cluster-moving Monte Carlo method. The internal structure of aggregates was analyzed quantitatively by means of the pair correlation function. The results obtained here are summarized as follows. Under the influence of a strong magnetic field, chain-like clusters are formed in the direction of the magnetic field and become longer and thicker as the magnetic interactions between particles increase. Also, thicker chain-like clusters are formed for a wider distribution of particle diameters. For the very weak external magnetic field, chain-like clusters are formed that have bend-like, small loop-like and branch-like structures. These structures are induced by the interactions between large particles. As the influence of the magnetic field dominates, these structures appear less and less, until straight chain-like clusters are formed in the direction of the magnetic field. The thickness of the clusters does not significantly depend on the strength of the magnetic field but on the distribution of particle diameters.