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To improve the thermal stability of starch in water-based drilling fluid, monoethanolamine (MEA) was added, and the effect was investigated by laboratory experiment. The experimental results show that the addition of monoethanolamine (MEA) increases the apparent viscosity, plastic viscosity, dynamic shear force, and static shear force of the drilling fluid, and reduces the filtration rate of drilling fluid and thickness of mud cake apparently. By creating hydrogen bonds with starch polymer, the monoethanolamine can prevent hydrolysis of starch at high temperature. Starch, as a natural polymer, is able to improve the rheological properties and reduce filtration of drilling fluid, but it works only below 121 °C. The MEA will increase the thermal stability of starch up to 160 °C. There is a optimum concentration of MEA, when higher than this concentration, its effect declines. Key words: monoethanolamine, starch, drilling fluid additives, water-based drilling fluid, thermal stability