Localizing a tiny fault causing abnormal leakage current in a large area P/N junction for a large MOS, diode or BJT structure by nano-probing was demonstrated. The localization was realized through probing the contacts on the junctions and comparing the reversed bias junction current for each contact which maintain the same polarity on the P/N junction. The tiny fault location which is causing the leakage in the large area P/N junction is indicated by the contact with the largest current due to the lowest sheet row resistance path to the fault as measured by nano-probing. Therefore, the success rate to identify the real physical fault by TEM and the ability to take the precise process action to correct the problem is greatly increased by this method. An artificial fault experiment induced by use of FIB as well as real case studies also verifies this method is valid and useful.