Abstract: A bacterial strain，designated as X3，which had the capability to degrade crude oil，was isolated from crude oil-polluted soil in the Dagang oil-field. The ability of strain X3 in the biodegradation of crude oil was studied，and the effects of different crude oil concentrations on the biodegradation of oil and the growth of bacteria were compared . The influence of different pH values and the concentration of NaCl on the biodegradation of crude oil were also investigated. The results showed that strain X3 was resistant to alkali and salt. The concentration of crude oil had a significant influence on the degradation rate of total petroleum hydrocarbons (TPH). When pH was 7 and the concentration of NaCl was 5g/L，X3 degraded approx. 72.6% of crude oil at 30℃ for 7 days. GC analysis showed that carbon number was the greatest factor that influenced the biodegradation. Degradation became more difficult with increasing carbon number of hydrocarbon.