100-nm-resolution at-wavelength holographic microscopy of aerial images produced by extreme ultraviolet (EUV) lithographic optics has recently been demonstrated. It provides a simple and compact method allowing image monitoring without printing in photoresist. Here, the concept of holographic microscopy is extended to the characterizations of defects on EUV multilayer mask blanks. As a proof of principle, defect characterization using the holographic microscope is demonstrated with programmed defects in transmission masks. Amplitude defects as small as 100 nm have been successfully characterized. Extension of this technique to the more relevant reflection mask configuration is also discussed.