External magnetic field and magnetic isotope effects on the lifetimes of several isotope-labeled triplet biradicals (BPH.-O-(CH2)(n)-O-BPH., n=8, 10, and 12) composed of two equivalent benzophenone ketyl radicals (BPH(.)s) were studied by laser flash photolysis in magnetic fields up to 14 T. The biradical lifetimes increased with increasing a magnetic field below ca. 3 T and then slightly decreased at 4-14 T. Below 4 T, a large magnetic isotope effect on the biradical lifetimes was observed in C-13 isotope substitution at a benzylic carbon of BPH., whereas no magnetic isotope effect was observed above 4 T. The magnetic isotope effect in magnetic fields up to 4 T can be explained only by the spin-lattice relaxation mechanism due to the anisotropic hyperfine interaction of the biradicals.