Uniformity of memory switching parameters is a critical issue in the application of resistance random access memory (RRAM). Herein, we report a resistive memory device based on a NiO layer pre-treated with an ionic liquid (IL) under positive voltage. Unlike a NiO-based device without IL pre-treatment, the IL-pre-treated device exhibited a forming-free characteristic, with uniform operation voltages and resistance as well as an enhanced R-off/R-on ratio during resistive switching. Further analysis indicated that performance improvement of the IL-pretreated device can be attributed to the formation of oxygen vacancies in the film and the creation of Ni-0-rich regions in the interface of NiO/Pt during IL pre-treatment. This finding offers a simple and practical pathway to develop high-performance filament-type memory devices.