Abstarct: The combination of open pit and underground mining; in particular determination of transition depth in large and massive deposits, is one of the most important issues in mine planning process and is challenged by mine engineers. In this case, a few solutions are available. therefore, due to these limitations in existing solutions, a new method baxsed on the economical block model with considering of optimum pit limit , geological and economical (mining costs & mextal price) uncertainties, applied to Chah-Gaz iron ore mine to determine transition depth from open pit to underground mining. For this reason, after estimation of grade distribution using Kriging method, 50 realizations generated by Sequential Gaussian Simulation method in order to investigation grade uncertainty. In addition, the influence of mining costs and mextal price uncertainties in transition depth utilized by Real Option approach and Binomial Tree method. baxsed on the results, the transition depth from open pit to underground mining for the case study is 350 meter by Kriging model and between 300 – 360 meters with considering of 95% confidence interval in grade uncertainty condition. Also, this depth is 410 meter from surface under economical uncertainties. The depth range that obtained by taking economical uncertainties is more than kriging and simulation models. It can be concluded that, the influence of economical uncertainties is greater than geological uncertainty in open pit mining.