GaAs surface cleaning prior to molecular‐beam epitaxy (MBE) using a new hydrogen radical beam produced by an electron cyclotron resonance plasma in an ultrahigh‐vacuum system is investigated. Residual oxygen and carbon on the GaAs surface after cleaning are monitoredinsituwith Auger electron spectroscopy. Oxygen and carbon, which are hard to remove by conventional thermal cleaning, can be removed by hydrogen radical (H*) beam irradiation at a substrate temperature of<400 °C. It is verified that the plasma‐dissociated radicals are much more reactive in the gas phase–solid phase surface chemical interaction than the nondischarged molecules. Ga and As stoichiometry is kept after H* beam cleaning. Good crystallinity is obtained according to reflection high‐energy elecron diffraction. By carrier concentration measurement at the interface between the cleaned surface and theinsituMBE‐regrown layer, it is found that H* beam cleaning reduces the interface state concentration. This cleaning technique makes clean GaAs surfaces available for MBE pretreatment.