Abstract: BL Lacertae object (BL Lac), a subclass of Active Galactic Nucleus (AGN), exhibit extreme observational properties: such as high luminosity and polarization, rapid variability, superluminal motion, radio core dominance, and highly energic emission. These observational properties are related to the beaming effect, which is quantified by Doppler factor. Doppler factor is helpful to understand the radiation physics and intrinsic characteristic of BL Lac. Based on the unified scheme of BL Lac and Fanaroff-Riley I/Fanaroff-Riley II (G), the author proposed that the distribution of intrinsic apparent magnitude vs redshift (Hubble diagram) for BL Lac should be similar to that of the observed apparent magnitude and redshift of FRI/FRII (G), and provided a method to estimate the optical Doppler factor of BL Lac. The researchers compared this Doppler factor estimation with the optical Doppler factor of the predecessor and found a close correlation between them. This result suggests that the method to estimate Doppler factor is reliable for depicting the beaming effect in the optical band of BL Lac.