Abstract: The on-line detection of fuel rod failure in nuclear power plant (NPP) can overcome the disadvantage of traditional chemical sampling method that could not monitor continuously and could not predict in time when rod failure occurred. An on-line detecting system (FDDS-1) for the failure rods in NPP was developed. By detecting the γ activities of fission products in the primary loop of NPP, the FDDS-1 could give the on-line analysis report of rod failure situation by the FUDAC-1 code.