  We study nonlinear optical response of Landau quantized graphene to an
intense electromagnetic wave. In particular, we consider high harmonic
generation process. It is shown that one can achieve efficient generation of
high harmonics with strong radiation fields -- when the work of the wave
electric field on the magnetic length is larger than pump photon energy. At
that high harmonics generation process takes place for a wide range of the pump
wave frequencies and intensities even for significant broadening of Landau
levels because of impurities in graphene.
