Abstract: Triaromatic steroids (TAS) may be formed by the aromatization of regular steranes. C₂₆-C₂₈ TAS widely occur in crude oils and source rocks. The C₂₆20R and C₂₇20S TAS typically co-elute with each other in m/z 231 mass chromatograms of aromatic fractions. Therefore, the relative abundance of C₂₆, C₂₇ and C₂₈ TAS cannot be determined by routine GC-MS analysis. A ternary diagram similar to that of C₂₇-C₂₈-C₂₉ regular steranes cannot be established to classify oil families, which leads to the limited use of TAS in petroleum geochemistry. This paper proposed a simple method to estimate the relative abundance of homologues and isomers of TAS on the basis of the distribution patterns, origin and thermal evolution of TAS isomers. Taking representative Paleozoic oil samples from the tectonic region of the Tarim Basin as an example, this paper distinguished two petroleum populations on the ternary diagram of C₂₆-C₂₇-C₂₈ triaromatic steroids determined by routine GC-MS analysis. The oils from the Fushan Sag in the Beibuwan Basin (South China Sea) have different distribution patterns of TAS. The oil family classification is also consistent with that by other molecular markers. For their relatively higher thermal stabilities, triaromatic steroids are generally present in significant abundance in oils and condensates with high maturity. Therefore, this ternary diagram of aromatic steroids is a potential geochemical indicator for oil-to-oil correlation and oil family classification as a substitute for that of regular steranes.