Incidence angle effect of a hydrogen plasma beam with an ion energy of about 20 eV was observed in a cleaning process for GaAs and Si surfaces for the first time. An atomically flat (001) GaAs substrate surface which was observed by clear Laue spots was prepared with a glancing angle of incidence. Similar improvement of smoothness was observed with the glancing angle of incidence on a Si surface when it was compared with perpendicular incidence. The mechanism is discussed considering momentum transfer parallel to the surface in the collision process and the resultant migration enhancement on the surface.