By means of the lattice-Boltzmann (LB) method, the mesoscaled structure of ferrofluids consisting of magnetic nanoparticles and a carrier fluid as well as some surfactant is investigated. The ferrofluid is a complicated system and its morphology is affected by a number of internal and external forces including gravitational force, Brownian force, van der Waals attraction potential, and dipole-dipole interaction potential. All these factors are included in the lattice-Boltzmann model. The distribution of suspended magnetic nanoparticles and morphology of the ferrofluid are simulated in both cases of the absence and the presence of an external magnetic field. The effects of the dipole-dipole interaction energy and the thermal energy on the aggregation structures of the magnetic nanoparticles are discussed. (c) 2005 Elsevier Ltd. All rights reserved.