  An empirical relation between the broad line region (BLR) size and optical
continuum luminosity is often adopted to estimate the BLR size and then the
black hole mass of AGNs. However, optical luminosity may not be a good
indicator of photoionizing luminosity for extremely radio-loud AGNs because the
jets usually contribute significantly to the optical continuum. Therefore, the
black hole masses derived for blazar-type AGNs with this method are probably
overestimated. Here we first derived a tight empirical relation between the BLR
size and the H$_\beta$ emission line luminosity, $R(\rm{light-days})=
24.05(L_{H_\beta}/10^{42} ergs s^{-1})^{0.68}$, from a sample of 34 AGNs with
the BLR size estimated with the reverberation mapping technique. Then we
applied this relation to estimate the black hole masses of some AGNs and found
that for many extremely radio-loud AGNs the black hole masses obtained with the
$R-L_{H_\beta}$ relation are systematically lower than those derived previously
with the $R-L_{5100\AA}$ relation, while for radio-quiet and slightly
radio-loud AGNs the results obtained with these two methods are almost the
same. The difference of black hole masses estimated with these two relations
increases with the radio-loudness for extremely radio-loud AGNs, which is
consistent with the fact that their equivalent widths of H$_{\beta}$ emission
line become smaller at higher radio-loudness. If the small H$_{\beta}$
equivalent widths of extremely radio-loud AGNs are indeed caused by the beaming
effect, we argue that the optical emission line luminosity may be a better
tracer of ionizing luminosity for blazar-type AGNs and the black hole mass
derived with the $R-L_{H_\beta}$ relation are probably more accurate.
