  The jet properties of Mrk 501 and 3C 279 are derived by fitting the broadband
spectral energy distributions with the lepton models. The derived gamma_b (the
break Lorenz factor of the electron distribution) are 10^4-10^6 for Mrk 501 and
200~600 for 3C 279 and the magnetic field strength (B) of Mrk 501 is usually
one order of magnitude lower than that of 3C 279, but their Doppler factors
(delta) are comparable. A spectral variation feature that the peak luminosity
is correlated with the peak frequency, which is opposite to the blazar
sequence, is observed in the two sources. We find that (1) the peak
luminosities of the two bumps in SEDs for Mrk 501 depend on gamma_b in both the
observer and co-moving frames, but they are not correlated with B and delta;
(2)the luminosity variation of 3C 279 is dominated by the external Compton (EC)
peak and its peak luminosity is correlated with gamma_b and delta, but
anti-correlated with B. These results suggest that gamma_b may govern the
spectral variation of Mrk 501 and delta and B would be responsible for the
spectral variation of 3C 279. The narrow distribution of gamma_b and the
correlation of gamma_b and B in 3C 279 would be due to the cooling by the EC
process and the strong magnetic field. Based on our brief discussion, we
propose that this spectral variation feature may be originated from the
instability of the corona but not from the variation of the accretion rate as
that for the blazar sequence.
