Abstract: Organic polymers and petroleum hydrocarbons are the main pollutants in viscous oil wastewater.The predominant strain B0501 was screened from viscous oil wastewater from Jincai Sewage Plant of Liaohe Oilfield to analyze the function of the strain on the COD_Cr removal rate in the biological treatment process of the viscous oil wastewater.The conditions including temperature,pH,hydraulic retention time and phosphorus and nitrogen nutrients affecting the COD_Cr removal rate of viscous oil wastewater by microorganism were investigated.Results showed that the inoculating of strain B0501 improved the COD_Cr removal rate.Under the conditions of 30 ℃,pH 7.5,hydraulic retention time 216 h and adding phosphorus nutrients meeting ρ(N)∶ρ(P) ratio of 5.63,the COD_Cr removal rate could be improved greatly,and ρ(COD_Cr) value could meet National Integrated Wastewater Discharge Standards(GB 8798-1996).Strain B0501 was identified as Aureobaterium liquefaciens according to their morphological,physiological and biochemical characteristics.