NiO films based extended gate field-effect transistor (EGFET) pH sensors were fabricated and investigated in this work. Experimentally, nickel oxide (NiO) nanoparticles were first prepared via precipitation method. Subsequently, the NiO film was deposited on the FTO glass substrate by spin-coating. In the precipitation of NiO, two precipitants, NaOH and NH4OH solutions, were used. The influences of preparation conditions including precipitant, coating number, and calcination temperature on the properties of NiO films and pH sensing performances of devices were investigated. From experimental results, it was found that the NaOH-derived NiO devices exhibited superior sensing performances than the NH4OH-derived ones, due to their smaller grain size and denser packing of NiO film. It also revealed that, a highest sensitivity of 53.40 mV/pH was achieved in the pH range from 2 to 12 with a good linearity of 0.9989, which was fabricated with a precipitant of NaOH solution, a coating number of 10, and a calcination temperature of 400°C. Moreover, the device showed a negligible hysteresis effect.