The effects of high magnetic field (less than or equal to 13 T) on the intramolecular exciplex fluorescence generated from a chain-linked phenanthrene (Phen) and dimethylaniline (DMA) system (Phen-(CH2)(n)O(CH2)(2)-DMA, n = 4-12) were studied in N,N-dimethylformamide by means of photostationary and laser-induced fluorescence. With increasing the magnetic field from 0 to ca. 1 T, the exciplex fluorescence for the molecules with n = 6-12 increases steeply in intensity and then decreases gradually up to ca. 9 T. Their mean lifetimes exhibit similar magnetic field dependence (less than or equal to 13 T). The reversal of the effects is observed in high magnetic field (> 1 T). This is interpreted in terms of the Delta g mechanism in which a high magnetic field enhances the singlet-triplet intersystem crossing in an intramolecular Phen anion and DMA cation radical pair intervening in the reaction. The Delta g value is estimated to be about 0.000 022.