The GaAs(001) 2X4 arsenic-stabilized surface and the selenium treated GaAs(001) surface grown by molecular beam epitaxy (MBE) have been investigated by scanning tunneling microscopy (STM). The results showed the differences between the structures of the surface after 30 ML GaAs growth (ML = monolayer) and that after growth of the GaAs layer. The GaAs(001) 2X4 surface unit cell was found to consist of two arsenic dimers and two missing dimers. We also observed the selenium treated GaAs(001) surface with a 2X1 reconstruction.