We have studied the magnetic domain structure of thin polycrystalline Co films by magnetic force microscopy. Cross-tie walls and a magnetic ripple structure within the domains have been resolved with high lateral resolution and sensitivity. The magnetization direction of the polycrystalline Co films have been determined locally by analyzing the position of the observed transversal ripple structure. By applying an external field in the plane of the sample along the magnetically easy axis we have observed a reversible rotation process of the ripple structure, whereas for an external field along the hard axis the magnetization process is characterized by domain-wall movement.