  We explore the relation between the linear length of radio core and the
central black hole mass for a sample of radio-loud active galactic nuclei
(AGNs). An empirical relation between the size of the broad line region (BLR)
and optical luminosity is used to estimate the size of the BLR. The black hole
mass is derived from $\rm H_\beta$ line width and the radius of the BLR on the
assumption that the clouds in BLRs are orbiting with Keplerian velocities. A
significant intrinsic correlation is found between the linear length of the
core and the black hole mass, which implies that the jet formation is closely
related with the central black hole. We also find a strong correlation between
the black hole mass and the core luminosity.
