Improving the crude oil properties can solve serious operational problems in oil production and reduce the production costs. In this study, the possibility of improving the quality of an Iranian heavy crude oil sample using two asphaltene-degrading bacterial consortia was investigated. So, two bacterial consortia were isolated from oil-contaminated soil and reservoir water samples and their impact on the composition, pour point, degrees API (The American Petroleum Institute gravity), and viscosity of crude oil at 30 and 60 degrees C after 14 days of incubation was assessed. The results showed that the pour point, degrees API, and viscosity of the samples decreased significantly. The maximum reduction in the viscosity of the crude oil at room temperature, 25 degrees C, was 95.73%, which is caused by isolated bacterial consortium from oil-contaminated soil and incubation sample at 60 degrees C. The results of gas chromatography-mass spectrometry analysis indicated that the amount of nC14 and nC18 in the Soil-60 and RW-60 samples and that of nC15 and nC16 in the Soil-60 and RW-60 samples increased. In addition, some complex polycyclic aromatic hydrocarbons spilled up simpler components, and the content of the simple aromatic hydrocarbon components increased significantly. Although the main criterion of the bacteria isolation was the ability to consume asphaltene, in the presence of lighter hydrocarbons, particularly saturated compounds, strains prefer to use them as carbon and energy sources and cause a decrease in the degrees API and viscosity of the crude oil. Consequently, the effect of these bacterial consortia on viscosity reduction of crude oil can improve the transportability of the crude oil in the well column and surface pipelines.