To clarify the relationship between nanostructures and magnetic properties of FeTiO(3)-Fe(2)O(3) solid-solution thin films, we have carried out dark-field transmission electron microscope (DF-TEM) and high-angle annular dark-field (HAADF) scanning transmission electron microscope (STEM) observations. The ordered-phase films show strong ferrimagnetic properties while the films identified as the disordered phase according to x-ray diffraction are weakly ferrimagnetic with high saturation fields, in contrast to completely disordered FeTiO(3)-Fe(2)O(3) solid solution for which antiferromagnetic properties or rather small magnetizations are expected. The DF-TEM and HAADF-STEM observations revealed that the ordered-phase films typically consist of cation-ordered domains of over 200 nm and that the Fe and Fe-Ti layers stacked alternately along the c axis, which leads to strong ferrimagnetic properties, are clearly distinguishable from each other. On the other hand, the films identified as the disordered phase are found to possess short-range ordered structure with antiphase boundaries distributed in cation-disordered matrix, rather than completely random cation distribution, explaining why the films are weakly ferrimagnetic with high saturation fields. The results demonstrate the significance of atomic-level observation of the cation distribution in this system for understanding the magnetic properties.