The shapes of aggregates of ferromagnetic ultrafine particles have been studied experimentally and theoretically. Experiment shows that the paricles form more chain-like aggregates when both the size and the saturation magnetization of the particles increase as far as their sizes remain within single domain. The chains become straightened under an external magnetic field. The primary particles in a single chain seem to have very narrow size distributions. The various shapes of the aggregates could be described mathematically by the concept of fractal’. The values of the efficiency factor which modifies the extent of Brownian coagulation were calculated from a model equation considering isotropic forces(van der Waals and hydrodynamic forces)and anisotropic force(magnetic force), respectively. It has turned out that the latter force dominates the formers in case of chain-forming particles and the model well supports the experimental observations on the chain linearization under the field and the monodispersity of the primary particles, described above.