Accurate estimation of annular friction pressure loss is necessary to perform drilling and well completion operations without lost circulation, pipe sticking or more serious well control problems. Determination of friction pressure loss for Newtonian and nonNewtonian fluids has been investigated in several experimental and theoretical works by considering the effects of eccentricity, pipe rotation or pipe geometry. However, there is a gap in the studies about the experimental investigation of temperature effect that is important especially in geothermal wells.This study experimentally examined the effect of temperature on friction pressure loss through vertical concentric annulus by using water and the polymer based drilling fluid including Polyanionic Cellulose and Xanthan Gum. Experiments were conducted in flow loop having 21-ft smooth and concentric annular test section (2.91 in ID casing x 1.85 in OD pipe). The effects of temperature on rheological model parameters, apparent viscosity, Reynolds number calculated with HerschelBulkley model having less error than Bingham Plastic and Power Law models for all temperature values were examined. It was found that consistency index and yield point were more sensitive to change in temperature than flow behavior index. Also, apparent viscosity decreased exponentially with increasing temperature and this decrease was more obvious in low shear rate values. Then, according to Reynolds number – temperature plot, earlier regime transition was observed with increasing temperature. As a result, increasing temperature caused the decrease in friction pressure loss, and temperature effect should be considered in future experimental and theoretical studies in order to estimate friction pressure loss in annuli precisely.