Abstarct: The mud weight is a key factor in a drilling operation. The difference between success or failure is nearly always tied to the mud weight program. Low mud weight may result in collapse and fill problems, while too high a mud weight may result in mud losses or pipe sticking. Borehole collapse occurs when the mud weight is too low because the hoop stress around the hole wall is very high, often resulting in rock failure (Aadnoy & Chenevert, 1987). The most important remedy is often to increase the mud weight. The mud weight window is one of the most important parameter in wellbore stability. So it is possible to minimize instability problems by determination of appropriate mud weight window. In this research, stable mud weight window (between pore pressure and minimum horizontal stress) is determined in a field in the south of Iran. Application of Information that where received from geophysics, petrophysics and geomechanic as input data and a specific failure criteria, calculated the collapse pressure and fracture pressure gradient. Two types of failure criteria where used in this research. The first is Mogi-Coulomb for vertical well. The second is Modified Lade criterion. Both of these criteria considered also effect of the intermediate principal stress. The mud weight window that estimated by two criteria is very closed together. The sensitivity on the minimum horizontal stress shows that this parameter is the most important input factor responsible for the uncertainty in the mud weight.