This master thesis presents the technology of rotary drilling as well as the role of drilling fluid in the process. Drilling fluid circulation system is also explained in this thesis. The functions and necessary properties of drilling fluids which are essential for the construction of deep geothermal well have been treated. Drilling fluid additives and their impact on drilling fluid properties is presented and well explained. The procedures for measuring drilling fluid properties and their corresponding measuring equipment have also been described. As a real case, a geothermal well of depth 2400 meters was purposed. Recipe for drilling fluid with ingredients for all the intervals was designed for individual intervals of drilling. At the end is a table that summarises quantities of necessary additives. The main objective of thesis is to show how to optimally choose drilling fluid for deep geothermal well completion. For the purpose of this study, a water based drilling fluid with gypsum polymer additive was chosen. This was selected because, it can fulfil all the necessary functions just as good as traditional bentonite drilling fluid, or even better sometimes. The main advantage of gypsum polymer drilling fluid is its good stabilisation of unstable shales and clay layers in formations. There are also some disadvantages associated with using polymer drilling fluid. The main limitation here is temperature because most polymers are only stable until 120 °C, but by adding specific co-polymers, the stability of polymer mud can be increased to at least 162 °C. This draws the conclusion that water-based gypsum-polymer drilling fluids are suitable for drilling deep geothermal wells as deep as 3000 meters in north-eastern Slovenia.