Mud filtrate invasion is considered as one of the most common sources of formation damage problems in naturally fractured reservoirs. The pore throat aperture is usually believed to be in a consistent trend of change in conventional reservoirs, while fractured reservoirs cannot be dealt with as above since the existence of the fracture. Underbalanced drilling is a drilling operation in which the hydrostatic head of the mud column is maintained at a pressure less than that of fluid in the porous medium. While the well pressure is stabilized at a constant amount lower than the formation pressure, the filtrate invasion can easily occur due to the capillary pressure and fracture opening wideness. As it goes, the same difficulties of conventional reservoirs may be seen in a wider and more intense amount. In this study, there is an endeavor to model the fluid flow behavior of reservoir fluid in the naturally fractured reservoir in order to predict the amount of filtrate invasion into the matrix medium. The modeling is derived according to the validated naturally fractured reservoir models; meanwhile, the hand-developed simulator is run to account for any possible changes of different parameters and concerning the correspondent effect on the final results. The results show a considerable invasion in the case of strong capillary pressure. What seems to be the most determining criteria are the temporary overbalanced and static drilling fluid situations, which in turn they will increase the extent and amount of drilling filtrate invasion. Also, the results show a harsh noticeable damage with low under balanced drilling pressure difference while high under balanced drilling pressure difference leads to nowhere.