The electrochemical behaviors of several chlorophenols were investigated on boron-doped diamond thin film electrode. 2-chlorophenol, 2,4-dichlorophenol, 2,4,6-trichlorophenol and pentachlorophenol solutions were prepared as the electrolyte. Such four chlorophenols with various number of chlorine atoms in the benzene ring were studied by cyclic voltammetry at different scan rates between –1V to 2.5V (vs. MSE). The results indicated that the reactions of the 2-chlorophenol, 2,4-dichlorophenol and 2,4,6-trichlorophenol at the diamond electrode are irreversible and under diffusion control. The irreversible trend increases as the number of the chlorine atoms decreases. However, the activity of the boron-doped diamond film electrode is not affected by the number of the chlorine atom. Different from the three chlorophenols, fouling of the electrode appears in pentachlorophenol solutions.