Three-dimensional Monte Carlo simulations have been carried out in order to investigate the thick chainlike fluster formation of ferromagnetic fine particles in an applied magnetic field, The results of the aggregate structures are summarized as follows. If a magnetic field is sufficiently strong and magnetic particle-particle interactions are relatively strong, thick chain like clusters are formed along a field direction. The thick chain like clusters obtained by the simulations agree well with those observed experimentally, For the increased particle-particle interactions, high potential barriers arise between thin chainlike clusters, so that it is impossible for the thin chainlike clusters to aggregate to form thick chainlike clusters. In small magnetic fields, thick chainlike clusters are not observed, since nonlinear clusters are preferred to linear clusters in this situation, whereby strong attractive forces do not act between clusters, due to flux closure.