The alkane and aromatic fraction of eleven biodegraded oils and five non-biodegraded oils from the Dongying Depression, Bohai Bay Basin, Eastern China were analyzed by GC-MS to investigate the effects of biodegradation and thermal maturation on the distributions of steranes. The results indicate that the biodegradation and depletion of steranes emerged in the heavily biodegraded oils (scale≥6). The oil samples show an increase in the C₂₉sterane C₂₉20S/(20S+20R) and C₂₉ββ/(ββ+αα) thermal maturity parameters in the heavily biodegraded oils, indicating this level of biodegradation(scale≥6) can alter the two maturity parameters. The selective depletion of C₂₇-C₂₉steranes follows the order  ααα 20R>αββ 20R>αββ 20S≥ααα 20S and C₂₇>C₂₉>C₂₈, and the diasteranes and C₂₀-C₂₁steranes were much more resistant to biodegradation than regular C₂₇-C₂₉ steranes.