Bicyclic Sesquiterpanes (BSs) are ubiquitous components of crude oils and ancient sediments. BSs in crude oils and diesel oil were identified and characterized, the effects of moderate weathering on BS distributions were discussed, and a methodology using diagnostic indices of BSs was developed for oil correlation and differentiation. The results showed that crude oils and oil products had different abundances and distributions of BSs and, consequently, resulted in different diagnostic ratios. The selected diagnostic ratio indices of BSs, such as BS4/BS5 (ratio of 4,4,8,9,9-pentamethyl-decahydronaphthalene to 8β (H)-drimane), BS6/BS5 (ratio of 4,4,9,9,10-pentamethyl-decahydronaphthalene to 8β (H)-drimane), BS8/BS9 (ratio of the second peak to the third peak of C 16 sequiterpane), and BS8/BS10 (ratio of C 16 sequiterpane to 8β (H)-homodrimane), still maintained better stabilities (%RSD < 5%) after weathering for 30 d. The longer weathering process (150 d) had some effect on such ratios (5% < %RSD < 10%). The facts of the uniqueness, abundance in petroleum, and chemical stability of BSs enable them to be suitable as an effective diagnostic means for identifying spilled oil with moderate weathering, particularly for lighter refined product samples that are difficult to identify by current techniques. BSs in crude oils and diesel oils were identified and characterized, the effects of moderate weathering on BS distributions were discussed, and a methodology using diagnostic indices of BSs was developed for oil correlation and differentiation. The results showed that crude oils and oil products had different abundances and distributions of BSs and, were, resulted in different diagnostic ratios. The selected diagnostic ratio indices of BSs, such as BS4 / BS5 (ratio of 4,4 8,9,9-pentamethyl-decahydronaphthalene to 8β (H) -drimane), BS6 / BS5 (ratio of 4,4,9,9- pentamethyl-decahydronaphthalene to 8β (H) -drimane), BS8 / ratio of the second peak to the third peak of C 16 sequiterpane) and BS8 / BS10 (ratio of C 16 sequiterpane to 8β (H) -homodrimane) still well better stabilities (% RSD <5%) after weathering for 30 d The longer weathering process (150 d) had some effect on such rati os (5% <% RSD <10%). The facts of the uniqueness, abundance in petroleum, and chemical stability of BSs enable them to be suitable as an effective diagnostic means for identifying spilled oil with moderate weathering, particularly for lighter refined product samples that are difficult to identify by current techniques.