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There are occasionally deposits with a potential of exploiting by a combined mining of open pit and underground. Hence, the most significant query emerges: where is the optimal transition depth from open pit to underground? In this study, in order to profitably select a single or combined mining of open pit and underground, as well as to determine optimal transition depth a methodology is presented. The model is established upon the economical block models of open pit and underground methods together with the Net Present Value (NPV) s gained by their mining. Main process in the model is economical comparison among several options of single and combined mining. To evaluate the model in detail, a hypothetical case is used. In this regard, among the all options a combined was selected that implying a height of 62.5 m as the optimal transition depth and the amount of 25.54 units of currency as the maximum total NPV achieved by the option.