The relative magnetization of an assembly of fine, ferromagnetic, single‐domain particles which show uniaxial magnetic anisotropy has been formulated as a function of &rgr; and &lgr;, the ratios of magnetic energy of particle moment in field and anisotropy energy to thermal energy, respectively, under various situations for particle orientation. Strict formulation for the system where the anisotropic symmetry axes of particles are distributed in thermal equilibrium gives the simple Langevin equation without any influence of anisotropy. In the case, however, of particles with fixed random orientation of symmetry axes the observed decrease of relative magnetization was deduced, while it was found that the Langevin equation is still valid as low and high field approximations. Several simpler cases where the symmetry axes of particles are constrained on a plane parallel or perpendicular to the applied field were also treated and the deviations from the Langevin equation were estimated. These results confirm Bean and Livingston's view about the behavior of the system in low and high field extremes, but show that Knappwost and Rust's formulation, which was developed to explain the anomaly in the magnetization of fine cobalt particles, is not reasonable, suggesting sources of anisotropy other than those of magnetocrystalline origin.