The MW irradiation can excite the electron single quantum transitions ("allowed transitions") when "ω"MW is close to "ω"e, resulting in a loss of the electron polarization. In addition, due to the small state mixing caused by the B term of the hyperfine interaction, it is possible to irradiate on the electron-nucleus zero quantum or double quantum ("forbidden") transitions around "ω"MW = "ω"e ± "ω"n, resulting in polarization transfer between the electrons and the nuclei. The effective MW irradiation on these transitions is approximately given by "Bω"1/2"ω"n.