A photo-Fries rearrangement reaction of I-naphthyl acetate in n-hexane Was Studied by a nanosecond laser flash photolysis technique under magnetic fields of 0 to 1.65 T. The escaped yield of 1-naphthoxyl radical decreased with increasing magnetic field (B) at 0 < 8 <= 0.005 T, but inversely increased with increasing B at much higher fields of 0.005 T < B <= 1.65 T. Such inverted magnetic field effects (MFEs) can be interpreted with the hyperfine coupling and the Delta g mechanisms through the singlet radical pair. The fact that MFEs are observed for the present photo-Fries rearrangement reaction suggests a singlet radical pair intermediacy with a lifetime as long as several ns.