The electronic structure of diamond C(111) surface during transition from (2×1) recons-truction to (1×1) structure induced by adsorbed hydrogens is studied using ab initio DV-Xα mathod in this paper. The result shows that the role of adsorbed hydrogen in the structural transition is firstly distorting the Pandey π-bond in the (2×1) reconstruction, then breaking the (SP2-hybrid + Pandey π-bond) structure and forming the sp3-hybrid bonds.