We measured the surface roughness of initially grown GaN layers on GaAs substrates in different precursor supply conditions. The surface morphology changed dramatically at low precursor flow even though the V/III ratio was constant. At low supersaturation, the number of grains decreased and the grain size enlarged. Therefore the surface was too rough to cover the GaAs substrate. A stepping-stone-like morphology occurred in extreme cases. A few cores grew abnormally large grains. Only nitridation occurred when supersaturation was too low to grow a GaN core. To cover the GaAs surface with a thin GaN layer, a fairly large precursor supply was necessary.