Abstarct: Abstract One of the most important recent studies by oil industry engineers to improve the drilling operations of wells, prevent drilling problems and ultimately the optimal production and operation of the reservoir is the study of geomechanics and wellbore stability. In this study, in addition to geomechanical modeling of the Asmari Formation in a well located in one of the Iran southwestern fields, a safe window of drilling mud weight was estimated in the entire sequence of this formation. First, the static and elastic parameters of the rock including bulk & yang module shear force, compressive and Poisson modulus for the whole sequence of Asmari Formation were calculated by using petrophysical logs (density logs, shear wave and compressive wave) and experimental relationships. Then the vertical stress was calculated according to the overburden laxyers and the desired depth. In the next step, the pore pressure diagram was calculated by using the information obtained from MDT log in the all section of Asmari Formation. The maximum and minimum horizontal stresses (Shmax & Shmin) were calculated by the information obtained from the previous steps and using the prolastic method which were calibrated with well evidence such as wellbore washout according to the caliper diagram. According to the obtained vertical and horizontal stresses, the fault regime was introduced in this normal sequence and at some land-slide intervals. As a result, a geomechanical model was constructed in this formation and at some selected intervals, sensitivity analysis was approch on it and the optimal drilling trojoctroy was determined according to the amounts of stresses obtained at these points finally optimal mud weight circolated. The average safe margin in Asmari Formation was between 7.9 to 15.2 ppg. The model obtained in this study can be examined with laboratory data and used in other wells that should be drilled in fild Mop.