The distributions of aliphatic and aromatic terpenoids in Cretaceous and Paleogene angiosperm fossils from Japan were investigated. Aliphatic A ring-degraded oleanane, ursane and lupane were found in all samples. In addition, aliphatic gymnosperm-derived diterpenoids were detected in significant quantities. Aromatic oleanane-type triterpenoids were the major components, with ursane- and lupane-type triterpenoids being minor compounds in all angiosperm fossils. In a Cretaceous gymnosperm fossil, major components included aliphatic labdane-, isopimarane- and abietane-type diterpenoids, aromatic abietane-type diterpenoids, and chrysene- and picene-type polyaromatic hydrocarbons (PAHs). Trace amounts of oleanane-, ursane- and lupane-type triterpenoids were also identified. It is likely that the aromatic triterpenoids were introduced to the gymnosperm fossil from the angiosperm-derived material in the sediments. We defined an angiosperm/gymnosperm index (al-AGI′) as an alternative to the aliphatic triterpenoid/diterpenoid ratio proposed in the literature. The al-AGI′ index may underestimate the angiosperm contribution, because the aliphatic angiosperm-derived triterpenoids are more readily converted to aromatic counterparts than the diterpenoids during diagenesis. An aromatic angiosperm/gymnosperm index (ar-AGI) can be calculated using aromatic diterpenoids and triterpenoids. We suggest that ar-AGI is best calculated using only the oleanane-, ursane- and lupane-type aromatic triterpenoids, and that the chrysene- and picene-type PAHs be excluded.