  The determination of the central black hole mass is crucial to the
understanding of AGN physics. In this paper we briefly review some methods that
are currently used to estimate the black hole mass of AGNs. Particularly we
demonstrate the importance of two correlations: one between the black hole mass
and the stellar velocity dispersion and another one between the broad line
region (BLR) size and the optical continuum luminosity. Besides applying these
relations in deriving black hole masses of various types of AGNs, we also
employed the fundamental plane of elliptical galaxies to estimate the central
velocity dispersions of AGN host galaxies, and then the black hole masses of
AGNs including BL Lac objects. In addition, we derived another empirical
relation between the BLR size and H_beta emission line luminosity from AGNs
with the BLR size measured by reverberation mapping studies, and argued that
more accurate black hole masses of extremely radio-loud AGNs could be obtained
with it than using the usual R-L_{5100\AA} relation because of the jet
contribution to the optical continuum. Finally we pointed out that black hole
mass estimation is very much helpful to determine the accretion rate and
understand the accretion process in AGNs.
