Chemical reaction of ethanol (CH3CH2OH) with Ge(100) surface has been investigated using scanning tunneling microscopy (STM) observation and density functional theory (DFT) calculations. At low coverage, high resolution STM images showed that ethanol dissociatively adsorbed on a single Ge dimer. The adsorption features included bright protrusions assigned to Ge-OCH2CH3 structure formed via O-H dissociation of ethanol. Real-time STM observations revealed that the molecular chain of ethanol increased gradually via successive adsorption along the dimer row direction following increased exposure to ethanol. DFT calculation results showed that the adsorption of ethanol on Ge(100) was dominated by kinetic control at room temperature. Thus, an integrated study of experimental and theoretical approaches coherently confirmed that ethanol reacts with Ge(100) via O-H dissociative adsorption and the final structure has the H-Ge-Ge- OCH2CH3 geometry on a single dimer of Ge(100).