We have investigated the adsorption structures and thermal desorption behavior of C2H4 on Ge(100) using scanning tunneling microscopy (STM) and temperature programmed desorption (TPD) under ultrahigh vacuum (UHV). Ethylene molecules adsorb in two distinct bonding geometries: (i) on top of a single Ge-Ge dimer (on-top) and (ii) in a paired end-bridge between two neighboring Ge dimers within the same dimer row (paired end-bridge). Real-time STM images taken during the exposure of C2H4 to Ge(100) show that the on-top configuration dominates over the paired end-bridge configuration. The TPD measurements show that chemisorbed C2H4 desorbs from Ge(100) nondissociatively with two different desorption features, denoted as alpha (385 K) and beta (405 K). Desorption follows first-order kinetics for both states; the desorption energies of the alpha (385 K) and beta (405 K) states are 1.05 and 1.15 eV, respectively. These desorption energies are about 0.6 eV lower than those of ethylene on Si(100), indicating that the Ge-C bond is weaker than the Si-C bond. STM measurements carried out after annealing Ge surface at various temperatures indicate that the alpha and beta states correspond to the on-top and paired end-bridge configurations, respectively.