Abstract: By means of 4 indigenous microorganisms separated from oil and water sam-pies of Jianghan Oilfield and of heavy oil reservoir of Liaohe Oillfield,the microbial action on the heavy oil sample of Wangxinxie 3,Jianghan Oilfield has been detailedly determined and analyzed. Comparing the oil components before and after the microbial action, it has been found that all the 4 kinds of bacteria can act on long chain alkane and aromatic hydrocarbon, produce large amount of surface active substances such as carboxylic acid,ester,ketone and alcohol simultaneously. Viscosity measuring indicates:the combined bacteria can largely reduce the viscosity of oil from Jianghan Oilfield and Nanyang Oilfield. Simulation tests indicate:all the 4 microorganisms or the combined bacteria can enhance oil recovery in some different degree.