By micromagnetic simulations and analysis we study domain size and magnetization distribution evolution under changes of magnetic fields applied parallel to ultrathin film, H-parallel to. Domain wall energies and domain periods were calculated for H-|parallel to = 0 and H-|parallel to close to the effective anisotropy field. The huge influence of the demagnetizing effect on the equilibrium domain period has been found. Simulated domain periods are in good agreement with those obtained analytically. (C) 2004 Elsevier B.V. All rights reserved.