Abstract: The characteristic ions (FW+1)~+, (FW-14)~+, (FW-30)~+ of 2, 2, 6, 6-tetramethylpiparidine nitroxide radicals (1-6) might be interpreted by(A)form of nitroxide radical. After ionization of (1-6) the ions are both nitrogenradical ion and oxygen radical. Authors suggested a mechanism of a methyl of2-or 6-position migrating to oxygen radical and the loss of CH_2O to interpretthe product of (FW-30)~+ in (1, 5, 6), but the (FW-30)~+ of (2, 3, 4) are pro-duced by the loss of nitroxide radical. It might reflect the competition of twomechnism each other. But the abundant fragment ions are supplied by fragmen-tation controlled by 4-substitute.