The biodegradation of crude oil in the reservoir is an important alteration process and has undesirable impact on the oil physical property and economic value. Most of the world oil has been biodegraded. The effects of biodegradation on the ordinary biomarkers have been well known. Some advances in the biodegradation of the uneasily biodegraded biomarkers in recent years, such as high-molecular-weight alkanes, tricyclic terpanes and 25-norhorpane, were reviewed. At present, the details and mechanism taking place during the biodegradation are still obscure. Both aerobic and anaerobic biodegradation mechanisms were discussed. It is proposed anaerobic biodegradation may be dominating and the biodegradation rate is very slow. The temperature was thought to be the main factor controlling the biodegradation. The biodegradation would cease at the reservoir temperature over 80℃. In most case, the biodegraded oils are mixing oils. The methods describing multiple hydrocarbon filling history of the biodegraded oil were introduced. Asphaltene of the biodegraded oil is unsusceptibility to biodegradation and its pyrolysate and Ruthenium-ion-catalyzed-oxidation production has important role in the oil-oil correlation and oil-source correlation for the biodegraded oil. Finally the trend of research on the biodegradation was proposed. 