Can lensed stars be regarded as pointlike for microlensing by MACHOs?

We derive an analytical formula for the magnification of an extended source with constant surface brightness lensed by a point mass. We find that the magnification for an extended source is significantly different from that for a point source when the impact parameter is comparable to the source size. In particular, the peak magnification for an extended source can be larger or smaller than the peak magnification for a point source. The results are relevant to the short-duration (day) and high- magnification events in the search of massive compact halo objects in the Galactic halo by microlensing.