  Gamma Ray Bursts (GRBs) are the strongest explosions in the universe which
might be associated with creation of black holes. Magnetic field structure and
burst dynamics may influence polarization of the emitted gamma-rays. Precise
polarization detection can be an ultimate tool to unveil the true GRB
mechanism. POLAR is a space-borne Compton scattering detector for precise
measurements of the GRB polarization. It consists of a 40$\times$40 array of
plastic scintillator bars read out by 25 multi-anode PMTs (MaPMTs). It is
scheduled to be launched into space in 2016 onboard of the Chinese space
laboratory TG2. We present a dedicated methodology for POLAR calibration and
some calibration results based on the combined use of the laboratory
radioactive sources and polarized X-ray beams from the European Synchrotron
Radiation Facility. They include calibration of the energy response,
computation of the energy conversion factor vs. high voltage as well as
determination of the threshold values, crosstalk contributions and polarization
modulation factors.
