The optical nonlinearities of semiconductor bulk material Indium nitride (InN) have been investigated by Reflection Z-scan technique (RZ-scan) at the wavelength of 532nm with 21ps laser pulse. Top hat beam is used in order to get high spatial quality. Different from the normal RZ-scan device, instead of moving the sample, the lens before the sample is moved along z-axis. This can guarantee the stability of the spot on the surface of the sample and the measurement accuracy. The measurement of nonlinear refraction without aperture should be tested first, and then with a small aperture nonlinear absorption coefficient is measured. The experimental results show that InN has positive nonlinear refraction and saturable absorption effect. Nonlinear refraction and absorption coefficient are obtained by fitting theoretical simulation with experimental data.