In this work we study the adsorptions of some small molecules or group on the hydrogenated C(100)-2×1 surface using density functional theory method. The calculated results show that the ionization potential (IP) of the hydrogenated C(100)-2×1 surfaces after adsorption has amphoteric characteristics. From the weak basic NH3 molecule with small IP and negative electron affinity (EA), through the neutral H2O molecule, to the weak acid HF molecule and the OH group with large EA and IP, the IP values of the adsorbed diamond surfaces vary from decrease, through invariability, to slight increase for HF and obvious increase for OH. In all adsorption species, only the OH group makes the hydrogenated C(100)-2×1 surface change to the metal from the semiconductor with a wide-band gap, while the others only introduce impurity states into the electronic structures of the hydrogenated C(100)-2×1 surfaces. In this work we study the adsorptions of some small molecules or group on the hydrogenated C (100) -2 × 1 surface using density functional theory method. The calculated results show that the ionization potential (IP) of the hydrogenated C (100) - From the weak basic NH3 molecule with small IP and negative electron affinity (EA), through the neutral H2O molecule, to the weak acid HF molecule and the OH group with large EA and IP, the IP values ​​of the adsorbed diamond surfaces vary from decrease, through invariability, to slight increase for HF and obvious increase for OH. In all adsorption species, only the OH group makes the hydrogenated C (100) -2 × 1 surface change to the metal from the semiconductor with a wide-band gap, while others only only impurity states into the electronic structures of the hydrogenated C (100) -2 × 1 surfaces.