In this paper, the methodology of modelling the production process of the underground hard coal mine together with the streams of financial outlays and benefits is characterised. The method was developed for the purpose of analysis and assessment of the economic efficiency of the investment works in terms of determining the output volume of the mine. The reference to the integrational method of the mine production process modelling and estimating its economic efficiency in the underground hard coal mine was made. On the basis of this reference, a model of the production process and associated with this process streams of financial outlays and benefits were built. Economic efficiency indices of the investment, as the objective functions were shortly characterised. The aspect of uncertainty and risk, associated with the process of planning the output volume of the mine were discussed. The most important information and data, assumed for the exemplary calculations, conducted in accordance to the developed methodology and the most important conclusions drawn from the calculations were also instanced.