The article describes a new analytical method using strain energy to predict the mud weight window (MWW) to safely use in oil well drilling design and operations. Derivation of MWW equation using strain energy, in both pre- and post-drilling states, is crucial to wellbore stability analysis and design. Thus at first, the determination of internal stored strain energy for two; pre- and post-drilling states have been formulated. Then, these formulas, based on strain energy balance, have been employed to wellbore wall. Accordingly, a method to determine the MWW has been developed and proposed. To evaluate the credibility of the present developed method for MWW determination, the result has been compared with the actual drilling mud pressure that was used in the field. Then, the results of this method has been compared with the existing MWW determination methods, which are based on failure criteria. The results of comparative analyses, have shown very good accord with both cases. Finally, it has been shown that when the pore pressure and horizontal in situ stresses’ difference is insignificant, the width of mud window becomes narrow. On the contrary, when the difference between pore pressure and horizontal stresses is significant, the width of the mud window becomes wider. In the latter case, the well remains stable in a wider range of mud pressure.