When a magnetic fluid thin film is subjected to a magnetic field perpendicular to the plane of the film, a portion of magnetic particles is condensed out from the initially monodispersed magnetic fluid and forms magnetic columns. These columns evolve from a disordered column phase to a two-dimensional ordered structural pattern as the field strength reaches some critical value, Hh. The structural pattern formative process of the ordered structures that follows depends on the condition of the disordered column phase. In the letter, the effect of the initial disordered column phase, which can be altered by changing the initial field strength, Hi, on the formative process of the ordered structures is studied, and a phase diagram on the H–Hi plane is constructed. With an initial field strength that exceeds Hh, the structural pattern formative process goes through a plateau, a transition phase, and another plateau. On the other hand, if the initial field strength is less than Hh, the columns in the disordered column phase split and new columns emerge in the magnetic fluid film as the field strength increases. Finally, the structural pattern reaches a plateau. In both processes, the distance between columns in the structural patterns converges to that of a pseudostatic case. © 2001 American Institute of Physics.