We demonstrate that the optical response of graphene nanoribbons in the terahertz to far-infrared
regime can be significantly enhanced and tuned by an applied magnetic field. The dependence of the
threshold frequency on the magnetic field is studied. The ribbons with the strongest terahertz
conductance under a magnetic field are those with one-dimensional massless Dirac Fermion energy
dispersion. For a given ribbon, there exists an optimal field under which the conductance resonance
can occur at the lowest frequency.