We have investigated the evolution of optical properties of nitrogen-ion-implanted GaAs wafer followed by rapid thermal annealing, using UV-VIS, infrared reflection and photoluminescence spectroscopies. Semi-insulating GaAs (100) wafer was implanted by 50 keV nitrogen ions for a fluence range of 0.5~4.0x10¹⁷ cm^-2 at different temperatures of room temperature, 500℃ and 700℃, followed by post-implantation annealing at 500~900℃ under pure nitrogen gas flow. In case of high-temperature implantation, there has been no significant change in the UV-VIS absorption spectra after high-temperature annealing compared with the spectra of the as-implanted sample. On the other hand, microscopic blistering and/or exfoliation is preferred after post-implantation annealing at high temperature above 600℃. As a consequence, low-temperature implantation (<200℃) is recommended in order to keep the morphologically clean sample surface especially at the implantation fluence of 2x10¹⁷ cm^-2 or more. Formation of nanometer-sized GaN crystallites could be confirmed by X-ray diffraction, X-ray photoelectron spectroscopy and cross-sectional transmission electron microscopy measurements, and the effects of different implantation and annealing temperatures on the evolution of the structure of crystallites were also described.