The electronic structures of monolayer silicenes (4 x 4 and root 13 x root 13R13.9 degrees) grown on Ag (111) surface are studied by scanning tunneling spectroscopy (STS) and density functional theory (DFT) calculations. While both phases have similar electronic structures around the Fermi level, significant differences are observed in the higher energy unoccupied states. The DFT calculations show that the contributions of Si 3p(z) orbitals to the unoccupied states are different because of their different buckled configurations.