The hydrogen abstraction of a benzophenone-phenol chain-linked compound was investigated in acetonitrile by means of a nanosecond laser flash photolysis technique at magnetic fields of 0-1.7 T. The Lifetime and the initial yield of a triplet biradical involving the benzophenone ketyl and phenoxyl radicals decreased with increasing magnetic field strength (B) from 0 to 0.005 T, but it increased with increasing B from 0.005 to 0.2 T. In a high-field region (0.2 T < B less than or equal to 1.7 T), the lifetime did not change appreciably, but the initial yield of the biradical decreased with increasing B. These anomalous magnetic field effects may be interpreted by the level-crossing mechanism at the low-field region and the Delta g mechanism at the high-field one.