  Magnetic fields play an important role in the evolution of molecular clouds
and star formation. Using the Velocity Gradient Technique (VGT) model, we
measured the magnetic field in Orion A using the 12CO, 13CO, and C18O (1-0)
emission lines at a scale of 0.07 pc. The measured B-field shows an east-west
orientation that is perpendicular to the integral shaped filament of Orion A at
large scale. The VGT magnetic fields obtained from 13CO and C18O are in
agreement with the B-field that is measured from the Planck 353 GHz dust
polarization at a scale of 0.55 pc. Removal of density effects by using a
Velocity Decomposition Algorithm can significantly improve the accuracy of the
VGT in tracing magnetic fields with the 12CO (1-0) line. The magnetic field
strength of seven sub-clouds, OMC-1, OMC-2, OMC-3, OMC-4, OMC-5, L 1641-N, and
NGC 1999 has also been estimated with the Davis-Chandrasekhar-Fermi (DCF) and
MM2 technique, and these are found to be in agreement with previous results
obtained from dust polarization at far-infrared and sub-millimeter wavelengths.
At smaller scales, the VGT proves a good method to measure magnetic fields.
