Low voltage driven capacitors, based on nickel oxide (NiO), were fabricated on the Pt/TiOx/SiO2/Si substrates by radio frequency magnetron sputtering. The effects of post-annealing conditions on preferential orientation, microstructure and tunable performance of NiO thin films were investigated. The NiO capacitors prepared at low annealing temperature possess large capacitance and the increasing trend is observed with the increase of bias voltage. For well-crystallized NiO thin films, the dielectric nonlinearity is observed after annealed at higher temperatures. The mechanisms leading to these dielectric responses are illuminated in detail. Finally, NiO thin film capacitors annealed at 300 degrees C present the highest tunability (68.1% @100 kHz) under a small applied field of 82 kV/cm. The dielectric performance reveals that NiO capacitors may be a potential candidate for tunable applications.