We have investigated aggregation phenomena of ferromagnetic colloidal dispersions composed of rod-like hematite particles which have a magnetic moment normal to the particle axis, by means of the cluster-moving Monte Carlo method. In concrete, we have treated a two-dimensional dispersion in order to clarify the influences of the particle aspect ratio, magnetic interactions between particles and the magnetic field strength on particle aggregations. The main results obtained here are summarized as follows. In the absence of an applied magnetic field, rod-like particles tend to aggregate to form raft-like clusters along the magnetic moment direction as magnetic particle-particle interactions increase. However, shorter raft-like clusters are formed as the area fraction decreases. If a strong magnetic field is applied, the raft-like clusters tend to incline along the magnetic field direction, and this feature of the cluster formation is not significantly dependent on the particle length.