The method that uses microorganisms to increase oil recovery from reservoir rocks is called Microbial enhanced oil recovery (MEOR). Using this method, the microorganisms are injected to the reservoir and produce the metabolic product which is used in MEOR. There are 6 main metabolic products (bio-products) that can be effected to the reservoir fluid and rock. The 6 main bio-products are bio-surfactant, bio-polymer, bio-mass, bio-solvent, bio-gases and bio-acid. Every bio-product has a different effect on the reservoir and is produced by different microbes, with its purpose being to decrease the residual oil saturation that is left behind in the reservoir rock. Bio-surfactant can reduce the interfacial tension between oil and the formation water and bio-polymers control the mobility of water that is used in waterflooding. Biomass can plug the reservoir pores and then change the flow direction of fluid flow in the rock. Bio-gas increases reservoir pressure and then forces the oil out of the rocks. Bio-acid can dissolve rock particles and open pore mouths, thus increasing rock porosity and permeability and allowing more fluid to flow, especially in Limestone. A MEOR field trial was successfully applied at ‘X’ Field X in Indonesia, involves 10 wells. It can decrease oil viscosity, increase in Oil API Gravity and decrease of the oil pour point. Oil production rate increased from 8 to 66%. Scale is observed as a disadvantage of successful MEOR. The results from this field trial demonstrate that MEOR is potential and offers some benefit to increase oil production.