A new eight-layer hexagonal perovskite Ba8NiNb6O24 was synthesized via the high-temperature solid-state reaction and its structure was characterized using selected area electron diffraction, high-resolution transmission electron microscopy, and synchrotron X-ray diffraction. Unlike the eight-layer ordered shifted Ba8CoNb6O24 and Ba8ZnNb6O24, Ba8NiNb6O24 adopts a twinned structure with stacking sequence (ccch)(2) for the BaO3 layers and displays more disordered cation and vacancies over the face-sharing octahedral (FSO) sites than the twinned tantalates Ba8MTa6O24 (M=Zn, Ni, Co). The stabilization of twinned structure and cation/vacancy ordering in Ba8NiNb6O24 composition is correlated with the smaller size difference between Ni2+ and Nb5+ in comparison with those between (Zn/Co)(2+) and Nb5+ in the shifted Ba8CoNb6O24 and Ba8ZnNb6O24. The Ba8NiNb6O24 pellet exhibits high dielectric permittivity epsilon(r)similar to 40, modest Qf similar to 41319GHz, and large temperature coefficient of resonant frequency (f)similar to 60ppm/degrees C. The lower Qf value compared with the high-Q Ba8MTa6O24 is ascribed to the reduced short-range B-cationic ordering inside the FSO dimers in Ba8NiNb6O24. These results contribute to understanding the interplay among chemical composition, structure, and dielectric properties of the eight-layer twinned and shifted hexagonal perovskites.