Cesium adsorption was investigated by means of thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES) and metastable de-excitation spectroscopy (MDS) in order to clarify why only a small amount of Cs is adsorbed on H-terminated diamond at room temperature. H-termination is not perfect and H-patches are formed on diamond surface. At room temperature this H-terminated surface can adsorb no Cs atoms and only bare diamond surface catches them. Therefore only a small amount of Cs is adsorbed on the H-terminated diamond surface at room temperature.