  We assembled a sample of Seyfert 1 galaxies, QSOs and Low-Luminosity Active
Galactic Nuclei (LLAGNs) observed by ASCA, whose central black hole masses have
been measured. We found that the X-ray variability (which is quantified by the
``excess variance'' sigma^2_{rms}) is significantly anti-correlated with the
central black hole mass, and there likely exists a linear relationship
sigma^2_{rms}\propto M^{-1}_{bh}. This can be interpreted that the short
time-scale X-ray variability is caused by some global coherent variations in
the X-ray emission region which is scaled by the size of the central black
hole. Hence, the central black hole mass is the driving parameter of the
previously established relation between X-ray variability and luminosity. This
findings favor the hypothesis that the Narrow Line Seyfert 1 galaxies and QSOs
harbor smaller black holes than the broad line objects, and can also easily
explain the observational fact that high redshift QSOs have greater variability
than local AGNs at a given luminosity. Further investigations are needed to
confirm our findings, and a large sample X-ray variability investigation can
give constraints on the physical mechanism and evolution of AGNs.
