The magnetic domain structure of lithographic Co line (1μm×280μm×70 nm) arrays was investigated in terms of a shape effect. The strong shape anisotropy along the Co line plays an important role in the formation of the magnetic domain structure. The magnetic stray field distribution above the Co line array was measured by using magnetic force microscope (MFM). As the magnetization of the Co line was saturated along the line, the stripe domain structure was observed. The direction of the magnetic moment was periodically modulated along the mean magnetization direction. The periodicity (λ= 0.8μm) is much longer than that of the magnetic ripple structure (∼10 nm) which is generally observed in the polycrystalline ferromagnetic Co thin film. The stripe domain structure is caused by the competition between the crystalline anisotropy perpendicular to the film plane and the shape anisotropy of the line.