We estimate the masses of massive black holes in BL Lac objects from their host galaxy luminosity .
The power of jets and central optical ionizing luminosity for a sample of BL Lac objects are derived from their extended radio emission and the narrow-line emission , respectively .
The maximal jet power can be extracted from a standard thin accretion disk/spinning black hole is calculated as a function of dimensionless accretion rate \dot { m } 
( \dot { m } = \dot { M } / \dot { M } _ { Edd } ) .
Comparing with the derived jet power , we find that the accretion disks in most BL Lac objects should not be standard accretion disks .
For a pure advection dominated accretion flow ( ADAF ) , there is an upper limit on its optical continuum luminosity due to the existence of an upper limit \dot { m } _ { crit } on the accretion rate .
It is found that a pure ADAF is too faint to produce the optical ionizing luminosity of BL Lac objects derived from their narrow-line luminosity .
We propose that an ADAF is present in the inner region of the disk and it becomes a standard thin disk in the outer region in most BL Lac objects , i.e. , ADAF + SD ( standard disk ) scenario .
This ADAF + SD scenario can explain both the jet power and optical ionizing continuum emission of these BL Lac objects .
The inferred transition radii between the inner ADAF and outer SD are in the range of 40 - 150 ~ { } GM _ { bh } / c ^ { 2 } , if the disks are accreting at the rate \dot { m } = 0.01 .