The chemisorption states of atomic hydrogen on Si (111) surface have been studied by means of thermal desorption spectra. Two desorption peaks β1 and β2 were observed. At low hydrogen exposure only peak β1 could be seen. The peak β2 appeared at higher hydrogen exposure and could not exist if the sample was exposed to hydrogen at a temperature higher than 400℃ or was annealed above 380℃ after hydrogen exposure. The saturated coverage of β2 was less than that of β1 by a factor of 2. The peak β1 is related to the top site adsorption of H on surface Si atoms as reported by previous works. The relatively high activation energy of desorption for β2 implies that its adsorption state is likely to be a chemical, bonding state instead of a non-bonding state as postulated by Schulze and Hender. All the above desorption behaviors of β1 and β2 are quite similar to those of monohydride phase and dihydride phase of H on Si (100) surface. Therefore, it is reasonable to deduce that the β1 and β2 states correspond to the monohydride and dihydride phase on Si (111) respectively. A triangle dimer stacking fault model of Si (111) (7×7) reconstruction proposed by McRae could be used to give an exp-laination of forming surface silicon dimer bond and thus the dihydride phase on clean Si (111) surface.