An in situ electrochemistry-electron spin resonance (ESR) method was established to study the generation reaction and the variation regularity of hydroxyl radicals (·OH) on a boron doped diamond (BDD) film electrode in aqueous solution. Results indicate that, above the oxygen evolution potential of BDD filmelectrode (2.4 V in 0.5 mol·L-1 H2SO4 solution), the generation rate of·OH increases as the applied potential or current density increases. However, there was no·OH ESR response when the applied potential was below 2.4 V. Compared to H-terminated surface, the O-terminated surface has a higher·OH generation efficiency because it is hydrophilic and this favors the water splitting reaction. The degradation processes for organic pollutants are always operated under high potential or current density, which allow the BDD film electrode surface to maintain its O-terminated condition. These conditions favor the·OH generation reaction and the reaction takes place with high activity. The pH value of the solution also influenced the·OH generation reaction. BDD filmelectrode has a stronger·OH generation ability in an acidic medium than in a neutral or alkaline medium. We also found that O-·3 can be produced on the surface of BDD film electrode. This article provides new insights into the mechanism of·OH generation on the surface of BDD film electrode and evidence of a highly efficient electrochemical oxidation process during the treatment of organic pollutants.