Adsorption and desorption processes of bismuth on Si (100) surfaces were studied using quadrupole mass spectrometry, LEED, and AES in an UHV system. At high substrate temperatures, only a two-dimensional phase with saturation coverage was formed on a surface, and the LEED pattern from this surface shows Si (100)2×1 structure with weak fractional order spots. At low temperatures, formation of the second phase was observed after the completion of the first phase. The results of LEED and AES experiments suggested that the second phase consisted of threedimensional islands. The peak of flash desorption spectrum for the second phase shifted to the higher temperature side with the increase of the initial coverage of bismuth. This phenomenon can be explained by considering the emitting processes of Bi atoms from the three-dimensional islands to the substrate surface and vacuum.