Electrogenerated chemiluminescence (ECL) of luminol at a boron-doped diamond electrode has been used for hypochlorite determination. The presence of H2O2 induces the generation of the ECL signals of luminol. In contrast, the presence of hypochlorite oxidizes luminol directly to decrease the ECL signals of luminol. Accordingly, a decrease of the ECL signals of luminol in the presence of H2O2 was used as the signal response for hypochlorite detection. A linear decrease of ECL signals with the NaClO concentration in the range from 0 to 20 μM was observed with a sensitivity of 18.56 a.u. μM−1 cm−2. An estimated detection limit of 0.88 μM was achieved, which is around one order lower than the detection limit obtained using the normal electrochemical method with the same electrode. The system also provides a good selectivity towards Cu2+ and Na+. A reproducibility of 3.40%RSD was noted for 15 repetitive measurements. The analytical performance was found to be favourable in comparison to those of other typical electrochemical and electrochemiluminescence methods, indicating that it is applicable for real sample detection.