A careful decision needs a comprehensive and effective way to estimate the subsurface condition and the possible resource contained in the geothermal system. Numerical model and simulation sum up the result of geoscience study and its correlation to the behavior of the subsurface fluid flows. A well-developed numerical simulation could give a thorough analysis of a geothermal system and be utilized as input parameters to apply the resource assessment method. This study applied the numerical simulation to the Ungaran geothermal field using TOUGH2 reservoir simulation software. Several parameters were referring to these simulation results and used as the input parameters to the resource assessment method using Monte Carlo simulation. The result shows that the resource value with the highest confidence is lower than the earlier resource estimation of the Ungaran Geothermal System, which is only based on geoscience study. This study gives a careful analysis to support decision-making, especially in a geothermal green field.