The capability of drilling fluid to endure high temperatures is an important characteristic of drilling fluids. Natural polymers are commonly used as additives for drilling fluids to improve their rheological properties. Starch is one of them, that is extensively used in the drilling due to economics. However, the thermal stability of water-based mud included starch is low. Herein, diethanolamine is used to enhance the rheological and filtration loss properties of the water-based fluids (WBF) through chemical modification of Lewis acid sites in bentonite framework and the formation of a cross-linked hydrogen bond with the starch compound. The effect of starch as an additive to WBF was studied at 0.56% and 0.94% in the bentonite-based fluid. The revealed data indicated an apparent improvement had been achieved after the addition of starch (0.94%). The effect of diethanolamine (DEA) on the rheological properties of starch-bentonite system, DEA was used as an additive at three ratios of 0.19%, 0.37%, and 0.75, at 300 °F, 325 °F, 350 °F, and 375 °F. The obtained data indicated that both rheological and filtration loss properties were enhanced, eventually at elevated temperature of 350 °F. The interaction in bentonite-starch, bentonite-DEA, and bentonite-starch-DEA WBF was studied using FTIR, XRD, and Raman spectroscopy. The results indicated the formation of hydrogen bonds between all components, which increases the basal displacing between bentonite layers. That interaction leads to the enhancement of the rheological and filtration loss properties of the drilling fluid. The response surface methodology (RSM) was used with the central composite design to evaluate and improve the drilling fluid compositions in terms of various rheological parameters.