Ultra Luminous X-ray sources (ULXs) are unusually X-ray bright non-nucleus point sources
in other galaxies whose luminosity exceed the Eddington limit of ordinary stellar mass
(~10 Mo) black holes (BHs). One possible interpretation of ULXs is intermediate mass
(100-1000 Mo) BHs (IMBHs) shining at sub/trans Eddington limit, and another is stellar
mass BHs shining at super-Eddington regime due to super-critical accretion flows.
Thanks to resent development in X-ray satellites, some new results were obtained from
several ULXs that support the IMBH scenario. In addition, the detection of a
gravitational wave from 60Mo BH have revealed that the IMBHs truly exist in the
universe. However, we must note that it is still ambiguous for most of the ULXs whether
they are truly the emissions from such massive BHs. In fact, some do show
characteristics expected in the super-critical accretion flow scenario. Most
surprisingly, the latest results revealed that some of them are not even BHs but
accretion-powered neutron stars, which ensures the existence of the super-critical
accretion flows.  Here, I would like to introduce the enigmatic natures of ULXs along
with resent observational results in X-ray.