Abstarct: In this study, the effect of bentonite on the rheological properties of the polymeric fluids and its effect on the formation damage and changes to the carbonate core permeability have been studied. Fluid samples were prepared from two types of polymers with two different sources that are commonly used in drilling fluids: Potato starch is a natural polymer; and Polyanionic Cellulose that known by the name of PAC in the drilling industry is considered modified natural polymer. The 3 different types of water (fresh water, sea water and NaCl saturated water) are used as the continuous phase of the fluid samples. In addition, the effect of temperature on the carbonate core permeability were investigated. 30 fluid samples (15 samples starch-containing and the other 15 samples PAC- containing) were prepared and tested. Rheological properties and fluid loss of the fluid samples, before and after the addition of various amounts of bentonite into the fluid composition, were measured and their corresponding graphs were plotted and then by comparison, the effect of the presence of bentonite and water is used as the continuous phase, on the behavior of the fluid samples has been investigated. To study the formation damage and to determine the effect of temperature, respectively, 12 and 4 cores carbonate is used. The results showed that the increase of Bentonite in the composition of the fluid samples, increases the rheological properties of the fluid samples; However, the fluid loss parameter in small amounts of bentonite, decreased and with increasing amounts of bentonite, the value of this parameter is increased. In addition, the salinity of the continuous phase of fluid sample does not have much impact on the performance of starch, unlike polyanionic cellulose. In case of the formation damage, revealed that presence of Bentonite in polymer fluid can greatly reduces the permeability of carbonate rocks and causes serious damages to it. It was also noted that the increase in temperature can considerably reduces the permeability of carbonate rocks to the polymeric fluid