Water trees are the most hazardous factor in the life of XLPE distribution cables and the major cause of insulation failure. It is well known that insulation failure causes stoppages in electrical service and requires extensive repair work. Therefore, deterioration of cable insulation has been diagnosed mainly by DC leakage current method. It is difficult, however to interrupt electrical service for diagnostic measurement. Several types of hot-line diagnostic methods (including the DC component current and DC superposition methods) are used to detect water tree deterioration. However, these methods have some problems, e. g., the effect of stray current, and the accuracy of measureent etc. On this account, we have been developing a new hot-line diagnostic method. We investigated whether the superposition of the voltage of various frequencies from the cover layer of energized cable and deterioration by water tree and the signal were correlated. As a result, we found that a deterioration signal of 1Hz was observed when we superposed the AC voltage (commercial frequency × 2+1Hz) on to the cover layer of cable where water tree had occurred in the insulation.