Drilling a well in safe and cost-effective manner is the main target of drilling engineers. Thus, wellbore stability, especially in highly deviated and horizontal wells, is the major obstacle in well design and, consequently, investing in many oil and gas fields around the world. These impediments are varied, such as lack of hole cleaning due to accumulation of splintered, blocky or tabular fragments, hydraulic hammers, tight holes, pipe sticking, lost circulation, poor cementing, sand production, reservoir compaction, surface subsidence and many others. All these problems may cause a significant increase in nonproductive time of drilling or production. The purpose of this thesis is to determine the safe mud window for a proposed horizontal well in one of the most stressful fields in the southern part of Iraqi oil fields.