Abstract: For crude oil, the influence of biodegradation is very important. Biodegradation of crude oil was simulated by aerophile bacteria, and the saturated hydrocarbon of the products was aralyzed by GC. It was shown that the n-alkanes would be biodegraded first, and the value of Pr/C₁₇ and Ph/C₁₈ would be increased, but the value of OEP and Pr/Ph would be kept no changes. When the isoprenoids (Pr, Ph) began to be biodegraded, the value of Pr/C₁₇ and Ph/C₁₈ would be reduced, and the value of Pr/Ph and OEP could not be measured or they were not the true value. After crude oil has been biodegraded, the parameters of saturated hydrocarbon in GC analysis should be changed. Lighter the crude oil is, faster will it be biodegraded.