As semiconductor process technology rapidly develops into deep-sub-micron or nanometer regime, the feature size of semiconductor devices continues to shrink down. As a result, the defect being able to cause a device malfunction is also becoming smaller and smaller, and even certain defect is invisible with high-resolution SEM or TEM. It makes conventional physical failure analysis (PFA) face a great challenge for deep-sub-micron processed devices and the PFA success rate decrease because of such tiny or invisible defects. Thus electrical failure analysis (EFA) is becoming more and more important. FIB circuit edit and electrical characterization can provide critical clues of the failure mechanism through diagnosing the behaviour of a suspected defective transistor even if the defect is invisible with high-resolution SEM and TEM. This paper has demonstrated its application in failure analysis for two cases of invisible defect issues