A user friendly and rugged online system has been developed based on Cerenkov photon counting technique for heavy water leak detection in steam generators of pressurized heavy water reactor. The system has the capability to identify presence of fission and activation products in the secondary coolant of steam generator, an indication of leak from primary coolant, by detecting Cerenkov emission produced by hard beta emitting radionuclides in water. In this method the steam sample from steam generator is cooled during its transit through sampling line and the condensate is passed online through a polythene plastic flow cell placed between two matched bi-alkali photomultiplier tubes with coincident counting circuitry to observe the Cerenkov photon count rate. The minimum detectable leak rate for the newly developed system, when used in a steam generator of a 220 MWe reactor, was determined to be 3.6 kg h−1. Upon designing, construction and characterization, the system was successfully employed to detect and identify leak in steam generator of a reactor.