Nitridation of GaAs(001) was performed using a radio frequency nitrogen plasma source in a molecular beam epitaxy chamber. The nitrogen-induced GaAs(001) (3 X 3) reconstruction was investigated by reflection high energy electron diffraction (RHEED) and X-ray photoemission spectroscopy (XPS). The temperature dependence of the (3 X 3) reconstruction as well as its evolution during further nitridation was observed by RHEED; we found that the (3 X 3) reconstruction could be obtained in the temperature range 400-580 degrees C by a very low dose of atomic nitrogen deposition. The nitrogen coverage in the (3 X 3) reconstruction is determined to be in the range 0.2-0.4 ML using XPS. We also discuss the atomic model of the (3 X 3) reconstruction and the energetics of its formation and stability.