Well (wellbore) stability is the prevention of brittle failure or plastic deformation of the rock surrounding the wellbore due to mechanical and chemical factors. Chemical instability usually occurs in shale formation, and strongly depends on composition of drilling fluid. But the most important mechanical parameters in wellbore stability include in situ stress, drilling fluid pressure(mud weight), pore pressure, and rock strength. Drilling fluid pressure(mud weight) is the only mechanical parameter that can be controlled. So due to prevention of wellbore instability, selecting the safe mud weight is necessary. If the mud weight is low, the shear failure will occur, and the wellbore instability is possible. On the other hand, if the mud weight is high, the tensile failure will occur. So calculating the window of mud weight which stabilize the well, is necessary. This window is named Safe Mud Weight Window(SMWW). In this research, the stability of Asmari formation of Marun oilfield is analyzed, and the SMWW is calculated by using FLAC3D software. Beginning of plastic flow on the wellbore wall was determined to specify the wellbore stability. Then the effect of rock strength properties, in situ stress, and pore pressure on SMWW have been assessed with sensitivity analysis. Based on the results, SMWW was obtained for different zones of Asmari formation in the oilfield under consideration. Also, the effect of geomechanical parameters is analyzed on wellbore stability. Consequently cohesion is more important than internal friction angle, and decreasing these factors (cohesion and internal friction angle) will increase possibility of instability. Changing in pore pressure and in situ stress(which are other geomechanical parameters) will affect wellbore stability too. Decreasing pore pressure cause the increase of SMWW. More closely in situ stresses around the well, the wellbore stability increase.