Functionalized diamond is promising for biosensing applications due to its attractive properties. Hydroxyl terminated diamond (OH-diamond) surface with chemically functionalizable carboxyl groups is beneficial to silanization, which creating an amine termination is necessary to immobilize proteins and DNA. However, fabrication of high quality OH-diamond surface is still difficult. In this work, OH-diamond is obtained by hydroxide ion treatments in deionized water at high voltages up to 3000 V. The OH-diamond exhibited high intensity of singly oxidized C_Ⅰ carbon bonds (COH and COC) and untraceable doubly oxidized C_Ⅱ carbon bonds (CO or OCO), characterized by X-ray photoelectron spectroscopy measurements. Meanwhile, OH and CO stretching vibration was observed in the Fourier transform infrared spectra. Furthermore, silanizaiton is achieved by 3-aminopropyltriethoxysilane (APTES) on the OH-diamond surface via condensation reaction with surface COH groups. In the end, the stability of OH-diamond surface was investigated by ReaxFF simulations and density function theory (DFT) calculations.