Abstract: In order to improve the economic benefits of oilfield development and further enhance oil recovery factor, mixed bacterial combinations were selected and field experiments of microbial enhanced heavy oil recovery were performed in 21 heavy oil wells in Kelamayi oilfield through single well huff and puff. The injected bacteria greatly lowered the viscosity of heavy oil and the deliver-ability of most experiment wells reached the economic level without steam injection. The return on investment of the experiments was relatively high. Experiment results show that the selected bacteria combinations can effectively act on heavy oil with high contents of colloid and as-phaltenes. The field experiments need a small amount of investment, can produce the desired results rapidly, and achieve high economic benefits. Therefore, microbial enhanced heavy oil recovery is worth of application and dissemination in Kelamayi heavy oilfield.