  Blazars are thought to possess a relativistic jet that is pointing toward the
direction of the Earth and the elect of relativistic beaming enhances its
apparent brightness. They radiate in all wavebands from the radio to the
gamma-ray bands via the synchrotron and the inverse Compton scattering process.
Numerous observations are performed but the mechanism of variability, creation
and composition of jets are still controversial.
  We performed multi-wavelength monitoring with optical polarization for 3C
66A, Mrk 421, CTA 102 and PMN J0948+0022 to investigate the mechanisms of
variability and research the emission region in the relativistic jets.
Consequently, an emergence of new emission component in flaring state is
suggested in each object. The most significant aspect of these results is its
wide range of sizes of emission regions from $10^{14}-10^{16}$ cm, which
implies the model with a number of independent emission regions with variety
sizes and randomly orientation. The "shock-in-jet" scenario can explain high PD
and direction of PA in each objects. It might reflect the common mechanism of
flares in the relativistic jets.
