A field pilot study has been constructed in the Liaohe oilfield,China to treat heavy oil wastewater with large amounts of dissolved recalcitrant organic compounds and low nutrient of nitrogen and phosphorus for improving its biodegradability by a upflow anaerobic sludge blanket (UASB).By operating the system for 252 days (including the start-up of 139 days),BOD5/COD ratio of the water increased from 0.176 to 0.252,the biodegradability improved obviously/remarkable,and the COD removal rate was 30.71%.Gas chromatography/mass spectrometry (GC/MS) was used to analyze the components of main organic surfactants indicating that the heavy oil wastewater contained 8 organic compounds are not the typical alkanes hydrocarbons but high molecular surfactants.After biodegradation 6 organic compounds were degraded,and 5 new organic compounds appeared,it maybe the degradation intermediates of some organic compounds.It indicates that the present biodegradation system is particularly effective at degrading large molecular organic substances but not for 2,2-methylenebis[6-(1,1-dimethylethyl)-4-methyl-Phenol and partly for Bis(2-ethylhexyl) phthalate.Environmental Scanning electron microscope (ESEM) showed that large quantity of filamentous microorganisms were immobilized on observed carriers.The UASB reactor appeared to be a useful option for the improving degradability and COD removal of heavy oil wastewater in Liaohe oil field.