The reaction mechanisms of ZrCl4 adsorption and dissociation on Ge/Si(100)-(2 x 1) surface as the initial stage of ZrO2 atomic layer deposition process are investigated with density functional theory (DFT). The Si-Si, Si-Ge and Ge-Ge one-dimer cluster models are employed to represent Ge/Si(100)-(2x1) surface with different Ge composition. The reaction of ZrCl4 with hydroxylated Ge/Si(100)-(2x1) surface forms a bridged ZrCl2 site, while on H-passivated surface, ZrCl3 site is formed. The reaction energy barrier of ZrCl4 with H-passivated SiGe surface is much higher than ZrCl4 with hydroxylated SiGe surface, which indicates that reaction proceeds more slowly on H-passivated surface than on OH-terminated surface. In addition, adsorption of ZrCl4 is favorable on Ge Surface atoms than on Si surface atoms.