We have investigated the influences of the magnetic interaction between particles and the aspect ratio of particles on aggregate structures in a colloidal dispersion composed of rod-like ferromagnetic particles by means of the cluster-moving Monte Carlo method. The internal structures of the aggregates obtained by simulations were analyzed in terms of the number density distributions Of Clusters and the radial distribution functions. The results obtained here are summarized as follows. As the magnetic interaction between particles increases, many small clusters such as pairs of anti-parallel particles, raft-like clusters, and small loop-like clusters are formed; these gather to form larger aggregates. In the case of a relatively strong magnetic interaction between particles, solid-like rectangular clusters are formed when the aspect ratio is approximately 2, since the suitable distance between magnetic charges enables particles to form a fundamental structure of normal pairs of anti-parallel particles. As the aspect ratio increases beyond 2, many more stable raft-like clusters are formed, since the increase in distance between magnetic charges makes the structure of the normal pairs of anti-parallel particles unstable. For a significantly larger aspect ratio, large network microstructures are produced by the formation of many chain-like and loop-like structures.