The nonlinear optical properties of pure ZnO and Ni-doped ZnO thin films are explored using the Z-scan technique at different input laser intensities and an excitation wavelength of 750 nm by 100 fs laser pulses. The pure ZnO and Ni-doped ZnO thin films were prepared by radio frequency magnetron sputtering at room temperature. A scanning electron microscope equipped with energy-dispersive x-ray spectroscopy was used to measure the thickness and composition of the thin films, while a UV-visible spectrophotometer was used to measure the linear optical properties. The structure of the thin films was measured using x-ray diffraction. Saturable absorption (SA) was observed in the pure ZnO thin film, while Ni-doped ZnO illustrated a combination of SA and reverse SA (RSA). The nonlinear absorption coefficient (β) and nonlinear refractive index (n2) of both pure ZnO and Ni-doped ZnO thin films were found to be input laser intensity dependent. As the input laser intensity increased, the nonlinear absorption coefficient and the nonlinear refractive index of both samples increased. An enhancement of two times in the nonlinear refractive index was observed for the Ni-doped ZnO thin film compared to the pure ZnO thin film. The optical limiting behavior of pure ZnO and Ni-doped ZnO thin films was investigated, and the data demonstrated that Ni-doped ZnO thin film is a good candidate for optical limiter applications due to the presence of strong RSA.