A new naphthoquinone, 5,8-dihydro-2,7-dimethyl-[1,4]naphthoquinone (1), which was named 5,8-dihysrochimaphilin, isolated from an ethyl acetate soluble fraction from the root of Pyrola japonica, together with chimaphilin (2). Compound 1 was transformed rapidly to 2 upon exposure to air by HPLC analysis. This fact supported that chimaphilin (2) may be an artifact from 1 .