The thermal desorption spectra of hydrogen chemisorpted on Si(100) clean surface have been measured by means of flash heating the sample under ultra high vacuum. The result shows that with the low hydrogen exposure at room temperature there is only one desorption peak A, while a second desorption peak B with lower peak temperature appears after increasing the exposure time. If the hydrogen exposures are carried out at elevated temperatures, the peak B does not appear in the desorption spectra at the temperature higher than 230℃, and no peak A appeared at the temperature above 530℃. With heat annealing the sample after room temperature exposure, the peak B will not existed if the annea- ling tempersture is higher than 350℃, and the peak A will also disappeared at 530℃ annealing. From all these features of thermal desorption, we believe that the peak A and B are related to the desorptions of monohydride and dihydride phase on Si (100) surface respectively. The measured activation energies of desorption of these two phases are 52.9 kcal/mol and 14.5 kcal/mol. It could be deduced from the peak asymmetry and the invariance of peak temperature with the initial hydrogen coverage that the desorption of peak A is happened as a first order desorption process. This has been futhur confirmed by the order plot. But the mechanism of hydrogen desorption on Si (100) surface is different from both the ordinary first order and second order desorption.