We reported a peculiar thickness dependence of perpendicular magnetic anisotropy (PMA), stronger in thinner films, in spinel NiCo2O4 (NCO) films deposited epitaxially on MgAl2O4 (001) single crystalline substrates. The effective PMA field is about 3 T in 23.0 nm-thick films but increases dramatically to about 7 T in 4.5 nm-thick films. X-ray absorption spectra reveal an inhomogeneous distribution of Co2+ cations along the film's normal with more Co2+ adjacent to the film/substrate interface. X-ray linear dichroism spectra reveal that the PMA in NCO films is associated with electrons occupying the x2–y2 orbital of Co cations at the tetrahedral sites. This work demonstrates the potential of ultrathin NCO films in all-oxide spintronic devices.