Abstract: As a low-cost, high stable hole transport material, nickel oxide has been widely used in inverted structure perovskite solar cells in recent years. By far, the most common method of preparing nickel oxide hole transport layers is spin-coating pre-prepared nickel oxide nanoparticles (NiO_x NPs), which puts forward high requirement for the particle sizes and solution processing capabilities of NiO_x NPs. In this work, the sizes of NiO_x NPs are precisely controlled by adjusting the pH value of the system in the synthesis process, and high-quality nickel oxide hole transport layers are then prepared. The experimental results exhibit that the NiO_x NPs with sizes of 5–10 nm are obtained at a pH value in a range of 9.5–9.8. More interestingly, the obtained NiO_x NPs have good dispersion stability and can achieve long-term dispersion in aqueous solution. Furthermore, the structural composition analysis of NiO_x NPs shows that the pH value of the synthesis system does not have a significant effect on the material structure nor composition of the NiO_x NP. Surface morphological analysis shows that the NiO_x film prepared by the pH-controlled NiO_x NPs is rather dense and particularly flat with small surface roughness. It is also found that the film exhibits good hole extraction capability. We also fabricate an inverted perovskite solar cell based on the NiO_x film. The device structure is ITO/NiO_x/CH₃NH₃PbI₃/PC₆₁BM/Bphen/Ag. It yields a good photovoltaic conversion efficiency (17.39%). In addition, the device is almost hysteresis-free. Our experimental results exhibit that the performance of perovskite solar cells can be effectively improved by precisely controlling the sizes of NiO_x NPs through pH values. Our work is expected to facilitate the development of NiO_x-based perovskite solar cells.