Under the controlled As partial pressure, the nitridation process of GaAs(001)-(2x4) surface was studied using a scanning tunneling microscope (STM) combined with an electron cyclotron resonance plasma-assisted molecular beam epitaxy system. With either prolonging the nitridation time or decreasing the As partial pressure, the previously reported (3x3) structure with two dimers per surface cell ((3x3)-2D) was found to progressively convert into a new (3x3) structure characterized by one dimer per surface cell ((3x3)-1D). Reversely the exposure to arsenic transformed the structure from (3x3)-1D to (3x3)-2D, suggesting that the topmost layer is composed of As 2 -dimers. Based on these STM images together with the X-ray photoelectron spectroscopy data, we propose the new As 2 -dimer coverage models to explain both (3x3)-1D and -2D structures involving the exchange reaction of arsenic with nitrogen in the subsurface region of GaAs.