A new method has been developed at Osaka University for producing a mega‐gauss field without destruction of the coil. As one of the applications of this system, submillimeter ESR experiments have been done at low temperatures by using HCN (&lgr;=337&mgr;m) and H2O (&lgr;=119&mgr;m) lasers in a field up to 0.5 MOe. We report some novel high field effects observed in these experiments. The g‐shift due to the SH3term is expected to be appreciable in such a high magnetic field. This effect was found in a study of Co‐Tutton salt at 77 K. The experimental result showed 2% smaller g value at 119 &mgr;m than at 337 &mgr;m. This was explained quantitatively by an extensive calculation based on the Abragam‐Pryce theory. So‐called exchange splitting was observed in (NH4)  2CuCl4⋅2H2O, Cu(NH3)  4SO4⋅H2O (CTS) and CuCl2⋅2H2O. At 4.2 K the two observed lines were of quite different intensities and the weaker one showed a remarkable shift. These phenomena can be explained successfully by molecular field theory, and it is shown from the direction of the shift that the exchange interaction between inequivalent spins is antiferromagnetic for CTS and ferromagnetic for the others.