The nitridation process of GaAs(001) surface was performed by exposing the surface of GaAs(001) to the flux of plasma-excited N

2

gas directly and indirectly in MBE system.Two processes show completely different effect on the GaAs surface.The nitridation performed by opening N

2

cell shutter resulted in deterioration of the GaAs(001) surface and formation of polycrystalline structure.With increasing of the N

2

pressure during the nitridation

the deterioration became serious.However

for the surface nitrided with the closed N

2

cell shutter

after nitridation process at 500℃

the(3×3) RHEED pattern was observed and the surface still kept atomically flat.Our results indicate that the nitridation (without opening N

2

shutter) of GaAs(001) surface at low temperature (500℃) is suitable for the formation of thin c-GaN layer with flat surface before normal GaN epilayer growth.