L10-type ordered FeNi alloy is a good candidate material for rare-earth-free permanent magnets due to its large saturation magnetization, large uniaxial magnetic anisotropy (Ku) and high Curie temperature. Previously, we investigated the fundamental properties of L10-FeNi by preparing single crystal films by alternate deposition of Fe and Ni monatomic layers. In the present study, we investigated the influences of the addition of Co on the magnetic and structural properties of L10-type ordered FeNi films. Three types of Co addition, with the substitution of Fe layers, Ni layers and both layers, were performed. X-ray diffraction using synchrotron radiation indicated that Co atoms were randomly distributed in films in all addition processes. A long-range order parameter of the L10-FeNi phase and Ku showed a tendency to decrease with the amount of Co for additions substituting Fe layers and both layers. However, Ku showed no decrease with Co addition substituting ⩽10% of Ni layers, even though the order parameter notably decreased. This indicates that an achievement of the Co addition substituting Ni atoms without any reduction of the order parameter enhances Ku in L10-FeNi.