  With the assumption that the optical variability timescale is dominated by
the cooling time of the synchrotron process for BL Lac objects, we estimate
time dependent magnetic field strength of the emission region for two BL Lac
objects. The average magnetic field strengths are consistent with those
estimated from core shift measurement and spectral energy distribution
modelling. Variation of magnetic field strength in dissipation region is
discovered. Variability of flux and magnetic field strength show no clear
correlation, which indicates the variation of magnetic field is not the
dominant reason of variability origin. The evolution of magnetic field strength
can provide another approach to constrain the energy dissipation mechanism in
jet.
