Molecular beam epitaxy (MBE) growth utilising an RF-plasma nitrogen source was used to study surface reconstruction and the effects of nitridation damage on the surface and interface morphology of GaN on GaAs(001) at 580 degrees C. Keeping both the N-flow and plasma excitation power constant, the grown layers were studied with the Ga-flux as a parameter. In the initial growth stage, a (3 X 3) surface reconstruction was observed. Samples grown under N-rich, Ga-rich and stoichiometric conditions were characterised by high-resolution scanning electron microscopy (SEM) and atomic force microscopy (AFM) to reveal the surface morphology. The nitridation damage was characterised by Auger-electron spectroscopy (AES), X-ray diffraction (XRD) spectroscopy and high-resolution SEM, which revealed separated GaN and GaAs phases at interfaces deeply in the substrate region, bordered by {101} and {111} facets. The defect formation at the GaN/GaAs interface depended on the N/Ga-flux ratio.