In this study, the efficiency of the native strain of Bacillus persicus in the process of microbial enhanced oil recovery was investigated by spontaneous imbibition and coreflood assay and its effective Mechanisms were evaluated. Also, in order to achieve the complementary results, all tests with culture medium, biological products and SDS surfactant were performed. The results of imbibition tests showed that the bacterial strain, bio-products and SDS surfactant recovered 12.5, 4.86 and 3.64% of initial oil in place, respectively. The observed enhancement oil recovery by the microbial solution was explained by variation of capillary pressure due to variation of wettability. The results of core flood experiments showed bio-products and microbial solutions were able to recover 4/82 and 4/77% of the initial oil in place in the tertiary oil recovery stage. These oil recovery enhancements were explained by the reduction of interfacial tension caused by the biosurfactant. Contact angle tests were performed to evaluate the ability of the solutions to change the wettability of carbonate rock surfaces. The results showed that the strain with wettability index of 80/97 changed the wettability of the carbonate surfaces from oil-wet to water-wet, while supernatant and surfactant solution with wettability indices of 54.66 and 59.06 changed the wettability to neutral state. Bio-surfactant production tests for assessment of the ability of the strain in reduction of the interfacial tension showed that this bacterium have a limited capability to produce biosurfactants and to decrease interfacial tension. but the utilized chemical surfactant had better performance than the produced biosurfactant. Increasing of the cores' porosity and permeability after imbibition tests confirmed that this bacterium acidify the environment by producing H2S, but the pH of the solution was slightly changed due to production of hydroxide ions that dissolve carbonate rock and consequently forming an equilibrium state in the solution. The viscosity of oil was decreased for the oil in contact the bacterial solution, biological product solution and culture medium (control test). But the viscosity reduction for the oil in contact with the bacterial solution and biological product had slight difference with the viscosity of the oil in contact with the culture medium. This result indicates that the bacteria did not use oil as a carbon source. Finally, the result of the bacteria's cell wall test showed that the strain, due to its hydrophilic characteristic can improve the wettability characteristic of carbonate surfaces to more water-wet.