The atomic layer deposition (ALD) of $ZrO_2$ was conducted in ultrahigh vacuum (UHV) conditions. The surface was exposed to $ZrCl_4$ and $H_2O$ in sequence and the surface species produced after each step were identified in situ with X-ray photoelectron spectroscopy (XPS). $ZrCl_4$ is molecularly adsorbed at 140 K on the $SiO_2$/Si(111) surface covered with OH groups. When the surface is heated to 300 K, $ZrCl_4$ loses two Cl atoms to produce $ZrCl_2$ species. Remaining Cl atoms of $ZrCl_2$ species can be completely removed by exposing the surface to $H_2O$ at 300 K followed by heating to 600 K. The layer-by-layer deposition of $ZrO_2$ was successfully accomplished by repeated cycles of $ZrCl_4$ dosing and $H_2O$ treatment.