Abstract: 25?norhopanes in hopane series compounds exist widely in severely biodegraded oils and are generally considered to be formed due to effect of natural biodegradation on the compounds, which has a special geochemical significance. The analysis of extracts from source rocks in different thermal evolution stages indicates that25?norhopanes are not contained in such extracts in low thermal evolution stage rather than high thermal evolution stage in which there exists high abundance of25?norhopanes in them. This paper suggests that this result is related to the thermal evolution degree, not just resulted from biodegradation. In Sichuan basin, the extracts from bitumen formed by high temperature pyrolysis all contain25?norhopanes, and the distribution features of them are very similar to the saturated hydrocarbon GC?MS diagrams for source rocks in high thermal evolution stage, and reach equilibrium state. Also, the hopane ratio parameter ranges in the same region, it can be deduced that the 25?norhopanes existed in the bitumen are effected by thermal evolution degrees. In order to make clear the differences between25?norhopanes occurrence related with the thermal evolution degree and resulted from degradation, the 25?norhopanes from biodegraded crude oil are compared with that from thermal evolution in distribution, it is verified that there exist large differences in GC?MS and hopane ratio parameter