Attempts were made to passivate the GaAs (001) surface by a pseudomorphic ultra-thin cubic GaN layer formed by a nitrogen radical (N-radical) or nitrogen plasma irradiation technique. Reflection high-energy electron diffraction (RHEED) pattern observations and detailed X-ray photoelectron spectroscopy (XPS) analysis have shown that ultra-thin cubic GaN layer on GaAs (001) surface with desirable surface stoichiometry can be realized with the optimization of surface nitridation process parameters. The passivation effects, characterized by ultra-high vacuum photoluminescence (UHV FL) analysis, revealed strong enhancement in band-edge PL intensity of GaAs after passivation as large as a factor of 10 when compared with the as-gown clean molecular beam epitaxy (MBE) GaAs surface.