The magnetization process in films of ferromagnetic superconductors is studied theoretically by concentrating mainly on the region near the lower critical field Hc1. Below a characteristic temperature, where the magnetic interaction energy plays an important role, the following are predicted: the unusual dependence of Hc1 on the film thickness, the discontinuity of the magnetization curve due to the change of the vortex lattice structure and the change of the transition type from II/1 to II/2 with decreasing the film thickness. The possibility of another type (II/3) of phase transition is also discussed. The mechanisms of these results are analyzed with particular attention to the roles of the interaction between each vortex and the magnetic field penetrating from the surface, the surface-affected intervortex interaction and the saturation effect of the magnetization which is mainly caused by the penetrating field. © 1986, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.