The structures of pteroside M and pterosin M isolated from the fronds of Onychium japonicum were reexamined by the NMR chemical shift reagent method. Furthermore, the stereostructure of pteroside M was established as 4-hydroxy-6-2'-hydroxyethyl-2 (R), 5, 7-trimethyl-1-indanone-2'-D-glucopyranoside on the basis of the circular dichroism Cotton effect associated with the n-π* transition of the conjugated ketones.