The experimental result shows that the preferred orientations of NiO thin films are closely related to the working pressure of argon. All of NiO(111), NiO(200), and NiO(220) diffraction peaks are observed in the XRD patterns and exhibited random orientation of NiO film when the film is deposited in low Ar pressure of 5 mTorr. As the Ar pressure is increased to 20 mTorr, only NiO(200) peak appears and shows (200)-textured NiO films. However, the lattice parameter of NiO film deposited in high Ar pressure of 20 mTorr is 0.424 nm, which is much larger than that of NiO bulk (0.417 nm). The lattice parameter can be decreased by post-annealing the film due to the interstitial Ar atoms are released from the NiO lattice and it reduces continuously from 0.423 to 0.417 nm as the NiO films are annealed from 300 to 600 °C. Therefore, post-annealing the NiO film deposited in higher Ar pressure will reduce the lattice misfit between the NiO and L10 FePt film that is beneficial to improve the perpendicular magnetic anisotropy of FePt film.