Abstract. In this paper, the central black hole masses and the Doppler factors are derived for PKS 0528 + 134, PKS 0537−441, 3C 279, PKS 1406−074, PKS 1622−297, Q1633 + 382, Mkn 501, and BL Lacertae. The masses obtained are in the range of (1 − 7) 10 7 M and compared with that obtained with the Klein-Nishina cross section considered (Dermer & Gehrels 1995). If we considered only the Thomson cross section, the masses are in the range of 2.6 10 6 M − 2 10 11 M . The masses obtained from our method are less sensitive to the flux than those obtained from Dermer & Gehrels (1995) method. The masses obtained from two flares (1991 and 1996 flares) of 3C 279 are almost the same. For 3C 279 and BL Lacertae, viewing angle, θ, and Lorentz factor, Γ, are estimated from the derived Doppler factor and the measured superluminal velocity. For 3C 279, θ = 10.• 9−15.• 6, Γ = 2.4−14.4 for δ = 3.37; θ = 8.• 45 − 9.• 7, Γ = 2.95 − 11.20 for δ = 4.89; for BL Lacertae, θ = 25.• − 29.• 4, Γ = 2.0 − 4.0.

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