Thermal oxidation treatment was executed on boron-doped nano-crystalline diamond (B-NCD) films to change both the microstructure and the terminal groups of grains and grain boundaries. Their contributions to the electrochemical activity and electrical properties were investigated. The grain boundaries become narrowing, with ordered graphite phase firstly, and then they become disordered for long time oxidized treatment. This causes that the mobility of the B-NCD electrodes increases to 84.97 cm(2)/V s after 30 min oxidation and then it decreases obviously with oxidation time further increasing. Nanocrystalline diamond grains with high carrier concentration are growing larger and naked on the B-NCD electrode's surface after oxidization. Also, part of the terminal C-H groups on the grain phases turn to C=O groups with positive electron affinity, which is prone to attract electrons from nearby anions. These results lead to larger electrochemical activity area with quick charge transfer rate in the surface of B-NCD electrode. The silane is used to shield the -OH sites from electrochemical activity. The covered area become less with the oxidized time, further confirming that the electrochemical reaction mainly occurs on the grain phases and the -OH groups is mainly growing in grain boundaries. (C) 2017 Elsevier Ltd. All rights reserved.