The alkane fraction of 11 biodegraded oils and five non-biodegraded oils from the Dongying Depression, Bohai Bay Basin, eastern China, were analyzed by gas chromatography-mass spectrometry to investigate the biomarker alteration caused by biodegradation. Results indicated that the concentration of 25-norhopanes was correlated with increased biodegradation. The oil samples showed an increase in the C-31 and C-32 hopane 22S/(22S + 22R), C-29 sterane C(29)20S/(20S + 20R), and C-29 beta beta/(beta beta + alpha alpha) thermal maturity parameters in the heavily biodegraded oils. Oleanane was preferentially biodegraded compared with C(30)17 alpha-hopane, which was preferentially biodegraded compared with C(29)17 alpha, 21 beta-norhopane, C-30 moretane, and C-29 25-norhopane. The selective depletion of C-27-C-29 steranes followed the order alpha alpha alpha 20R > alpha alpha alpha 20S + alpha beta beta 20R > alpha beta beta 20S and C-27 > C-29 > C-28, and the diasteranes and C-20-C-21 steranes were much more resistant to biodegradation than regular C-27-C-29 steranes. The steranes were generally preferentially biodegraded compared with the hopanes in this study.