We propose a method for directly detecting MASsive Compact Objects (MASCOs) in extragalactic halos, using VLBI techniques with extremely high resolution ˜ 0.01 milli-arcsec. If a galactic halo comprising a large number of MASCOs produces multiple images of a background radio-loud QSO by gravitational lensing, then a high-resolution radio map of each macro-lensed image should reveal microlensing effects by MASCOs. Based on the size, position, and magnified or demagnified patterns of images, we shall be able to determine the mass and density profile of an individual MASCO as well as its spatial distribution and abundance in a galactic halo.