Photoinduced electron transfer in a supramolecular complex of zinc(II) tetraphenylporphyrin-viologen covalently linked compound (ZnP-V2+) with 2,3,6-tri-O-methyl-beta -cyclodextrin in acetonitrile-water (1:1, v/v) was investigated. On increasing the magnetic fields, the lifetime of photogenerated biradical (ZnP.+-V.+) increased and reached an asymptotic value under high magnetic fields (> 100 mT). The effects were explained by the electron-nuclear hyperfine coupling and spin-lattice relaxation mechanisms. The observed forward electron transfer was quite slow compared with the free ZnP-V2+, and was increased by the magnetic fields. These results were explained by the fact that the photoinduced electron transfer was an equilibrium between the (ZnP.+)-Zn-3-V.+ and ZnP.+-V.+. and this free energy change was quite a small value (ca. -1.3 kJ/mol).