The limits on the mass of the black hole in 23 BL Lac objects are obtained from their luminosities of the broad emission line H \beta on the assumption that broad emission lines are emitted from clouds ionized by the radiation of the accretion disk surrounding a black hole .
The distribution of line luminosity L _ { { H } \beta } of all these BL Lac objects suggests a bimodal nature , although this can not be statistically proven on the basis of the present , rather small sample .
We found that standard thin disks are probably in the sources with L _ { { H } \beta } > 10 ^ { 41 } erg s ^ { -1 } .
The central black holes in these sources have masses of 10 ^ { 8 - 10 } M _ { \odot } , if the matter is accreting at the rate of 0.025 { \dot { M } } _ { Edd } .
For the sources with L _ { { H } \beta } < 10 ^ { 41 } erg s ^ { -1 } , the accretion flows have transited from standard thin disk type to the ADAF type .
The lower limits on the mass of the black hole in these sources are in the range of 1.66 - 24.5 \times 10 ^ { 8 } M _ { \odot } .
The results support the evolutionary sequence of blazars : FSRQ \rightarrow LBL \rightarrow HBL .