Plasma-enhanced atomic layer deposition (PEALD) is well-known for fabricating conformal and uniform films with a well-controlled thickness at the atomic level over any type of supporting substrate. We prepared nickel oxide (NiO) thin films via PEALD using bis(ethylcyclopentadienyl)-nickel (Ni(EtCp)2) and O2 plasma. To optimize the PEALD process, the effects of parameters such as the precursor pulsing time, purging time, O2 plasma exposure time, and power were examined. The optimal PEALD process has a wide deposition-temperature range of 100-325 °C and a growth rate of 0.037±0.002 nm per cycle. The NiO films deposited on a silicon substrate with a high aspect ratio exhibited excellent conformality and high linearity with respect to the number of PEALD cycles, without nucleation delay.