  We investigate the correlation between 151 MHz radio luminosity, L_151MHz,
and jet power, P_jet, for a sample of low-power radio galaxies, of which the
jet power is estimated from X-ray cavities. The jet power for a sample of FR I
radio galaxies is estimated with the derived empirical correlation. We find
that P_jet/L_Edd is positively correlated with Lx(2-10 keV)/L_Edd for FR Is,
where L_Edd is the Eddington luminosity and Lx(2-10 keV) is 2-10 keV X-ray
luminosity. We calculate the jet power of a hybrid model, as a variant of
Blandford-Znajek model proposed by Meier, based on the global solution of the
advection-dominated accretion flow (ADAF) surrounding a Kerr black hole (BH).
Our model calculations suggest that the maximal jet power is a function of mass
accretion rate and the black hole spin parameter j. The hard X-ray emission is
believed to be mainly from the ADAFs in FR Is, and the mass accretion rate is
therefore constrained with the X-ray emission in our ADAF model calculations.
We find that the dimensionless angular momentum of BH j>~0.9 is required in
order to reproduce the observed relation of P_jet/L_Edd-Lx(2-10 keV)/L_Edd for
FR Is. Our conclusion will be strengthened if part of the X-ray emission is
contributed by the jets. Our results suggest that BHs in FR I radio galaxies
are rapidly spinning, which are almost not affected by the uncertainty of the
black hole mass estimates.
