Heavy oil which is the largest potentially recoverable energy source faces many problems in recovery due to its high viscosity and heavy metals contents. Today there is an interest worldwide to develop new and clean technologies for heavy oil recovery. One of these new leading technologies is Microbial Enhanced Oil Recovery. Bacteria are able to live with oil and cause desired changes in its physiochemical properties. In this study, changes that have occurred in heavy oil from Mkhazina well after treatment with different bacteria consortia from Omani oil contaminated sites were investigated. Thermophillic spore forming bacteria from contaminated soil were inoculated with heavy oil in three different media; MO, M7 and M8. Bacterial growth, changes in Surface tension, Interfacial tension and viscosity of bacterial cultures in different days of incubation were measured. Bacterial growth patterns were found to be the same in all media types indicating that change in components not affecting growth. Clearing zones were formed in oily plate with all bacterial cultures and their free cell filtrates. The highest bacterial density was found at day twenty one of incubation with heavy oil. No significant decrease in surface tension and interfacial tension was found for all samples. This study showed changes in viscosity of bacterial cultures with heavy oil due to degradation of long hydrocarbons. Highest Viscosity of liquid medium was found at day seventeen after 21 days of incubation with heavy oil. Gas chromatography analysis showed difference in oil composition for crude oil and treated oil. Treated oil for 21 days with bacteria consortia showed presence of light hydrocarbons ranging from (C11-C27) that were not detected in non-treated heavy oil. 16 SRNA sequencing of 27 isolates from contaminated soil showed that isolates belong to three classes (Alpha-proteobacteria, gamma-proteobacteria and Bacilli). Scanning electron microscopy of oil utilizing bacteria showed that they were rod in shape with presence of rough surface and fimbria. In conclusion, these isolates may be useful in heavy oil recovery by changing its viscosity and oil composition.