We report a study of the effects of external magnetic field on the magnetic domain structure of a 250-nm-thick MnAs epitaxial film on GaAs (001). The surface topography and the magnetic-domain structures were investigated by atomic-force microscopy and magnetic-force microscopy, respectively. A self-organized periodic array of stripes, consisting of ferromagnetic alpha-MnAs and paramagnetic beta-MnAs, were formed during the deposition. A complex magnetic-domain structure appears in the ferromagnetic alpha-phase region at the demagnetized state (H = 0 Oe). As the magnetic field increases, the magnetic-domain structure changes gradually, and reaches a completely saturated state at H = 600 Oe. Especially, at H = 300 Oe, the observed magnetic domain-wall distribution matches well with the topographic one. (c) 2006 Elsevier B.V. All rights reserved.