Abstract: Two oils(one suffered obviously transformation,the other did not) from the Shengli oilfield were selected for anaerobic biodegradation in laboratory.The oils with microbiological degradation were regularly extracted and analyzed.It showed that the oil group composition was changed during anaerobic degradation,and CO₂ was produced and part of them was reduced to CH₄.The value of carbon isotope of the secondary CH₄ was mainly distributed in about -43.4‰,δ¹³C of CO₂ was mainly in -11.8‰.With the degraded time increasing,δ¹³C of CH₄ was become lighter and δ¹³C of CO₂ become heavier.