We have investigated the hydrogen desorption from a flat H-Si (111)1x1 surface at temperatures of 711, 730, 750 and 770 K by observing SFG and SHG spectra. The flat H-Si (111) surface was prepared by dosing hydrogen molecules in a UHV chamber. By using SFG, we detected Si-H vibration and investigated hydrogen desorption at high coverage from 1ML to〜0.29 ML, 0.30 ML, 0.43 ML and 0.44 ML. After SFG measurement, we switched to SHG measurement and detected Si dangling bonds and monitored the hydrogen coverage. Our investigation showed that the hydrogen desorption was second order in the coverage range from 1 ML to〜0.44 ML with an activation energy of Ed = 60 kcal/mol (2.60±0.16 eV) and first order in the coverage range from〜0.29 ML to 0.0 ML with an activation energy of Ed = 37 kcal/mol (1.61±0.11 eV). Keywords: Sum frequency generation (SFG); Second harmonic generation (SHG); Si (111)1x1surface; desorption order; desorption activation energy; hydrogen coverage.