Density functional theory was employed to investigate atomic layer deposition (ALD) mechanism of zirconia on Si(100)-2 X 1 surface with single and double hydroxyl groups. The ZrO2 ALD process using ZrCl4 and H2O as precursors involves two alternate deposition half-reactions: ZrCl4 and H2O half-reactions. Both the half-reactions proceeded through an analogous trapping-mediated mechanism. Compared to the singly hydroxylated Si surfaces, the neighboring hydroxyl of the doubly hydroxylated Si surfaces has a major effect on ZrCl4 half-reaction, especially the formation of ZrCl4 intermediate complex, and a minor effect on H2O half-reaction. Both ZrCl4 and H2O half-reactions were found to follow a similar reaction pathway with IRC method. In addition, the intermediate stability was lowered as the surface temperature was raised. However, increasing temperature also enlarged the dissociation free energy barrier, which in turn resulted in increased desorption of adsorbed precursors.