We grew Mn4-xNixN epitaxial thin films on MgO(0 0 1) by molecular beam epitaxy, as well as studied their crystalline qualities and magnetic properties. The films were decomposed into Ni8N or Mn-Ni alloys when x >= 2, as confirmed by X-ray diffraction and reflection high-energy electron diffraction, but this decomposition was mitigated by reducing the substrate growth temperature. The lattice constants decreased with increased Ni substitution except when the Mn ratio was high, while the crystal orientation tended to degrade. The magnetic properties were measured via vibrating sample magnetometer, and it was found that the saturation magnetization (M-S) and perpendicular magnetic anisotropy (PMA) diminished with a small amount of Ni substitution. Specifically, the M-S value was remarkably decreased from 86.3 +/- 1.1 emu/cm(3) (Mn4N) to 19.0 +/- 0.5 emu/cm(3) (Mn3.85Ni0.25N), and the magnetic anisotropy constant was decreased from approximately 0.94-0.027 Merg/cm(3), respectively. The PMA vanished with further Ni substitution. Ultimately, a small M-S and a PMA were simultaneously achieved with a small amount of Ni substitution. These properties support spin transfer torque, which can be applied to the emerging non-volatile memory devices using domain wall motion.