The reconstruction of diamond C(100) surface is studied with local density functional theory and extended numerical basis set. A cluster C49H36 is adopted to simulate the surface, where H atoms are saturation atoms. According to the calculation of forces exerted on the surface C atoms, an optimized C(100) surface atomic structure is achieved with the surface dimerized bond length, C-C, equals 1.415?. The nature of the bonding and the corresponding electronic structures, especially the distribution of surface states, are investigated and discussed.