In this paper, by employing conventional femto-second Z-scan and pump-probe measurements, the nonlinear optical properties and photoexcited carrier dynamics relaxation time of the GaN film were investigated. Based on the oscillation transmittance spectrum of GaN film and linear optical theories, the linear absorption coefficients (α0), linear refractive index (n0) and optical bandgap (Eg) were obtained at near ultraviolet wavelength of 370 nm. Variable nonlinear absorption effects were observed with the open-aperture (OA) Z-scan experiments at different levels of laser excitation intensities. It is found that the GaN film shows saturable absorption at low intensities and reverse saturable absorption under the excitation of intense irradiances when the photon energy is close to the band gap of GaN film. It is believed that the observed nonlinear absorption originates from the one-photon absorption at low intensities whereas one-photon induced free carrier absorption dominates the nonlinear response at high intensities. With the carry out of closed-aperture (CA) Z-scan experiments, a large nonlinear refractive index in GaN was obtained, which was one orders of magnitude larger than that of conventional nonlinear media. To identify ultrafast carrier dynamic relaxation time of the observed nonlinearities and to get an insight of the physical mechanism, the cross-polarized femto-second degenerate pump-probe measurement was performed at a near-ultraviolet wavelength of 370 nm. The results indicate that the saturable absorption originates from the instantaneous one-photon absorption process at low intensities, while one-photon absorption induced free carrier absorption dominates the non-instantaneous nonlinear absorption process at high intensities with the free carriers dynamic relaxation time ~17 ps. This work provides significant insight to the application of GaN in nonlinear optical ultraviolet photonics devices and the understanding of nonlinear response mechanism in GaN films.