Ultraluminous X-ray Sources (ULXs) are off-nuclear sources in other galaxies, which have luminosities in excess of that of a 10-solar mass compact object accreting at the Eddington limit. The question is still debated and unresolved, whether ULXs host intermediate-mass black holes (100-1000 solar mass), or stellar-mass black holes with highly beamed emission. I will review the existing methods to estimate the mass of black holes in ULXs, and present a new one. This new method is based on the recently discovered ?variability plane?, populated by Galactic stellar-mass black holes and supermassive active galactic nuclei, in the parameter space defined by the black-hole mass, accretion rate and characteristic frequency. I will apply this method to the two ULXs, M82 X-1 and NGC 5408 X-1, from which low-frequency quasi-periodic oscillations have been discovered. I will discuss the initial assumptions of the method, and discuss its high potential.