Abstract: Aiming at serious fluorine pollution problems in current hydrofluoric acid treatment processes of niobium-tantalum ore, a new leaching process of niobium-tantalum ore with an alkaline hydrothermal system was proposed that KOH solution was used as a reaction medium to substitute highly toxic HF solution. The reaction rules of niobium-tantalum ore in the alkaline hydrothermal system were investigated. It is found that decomposition of niobium-tantalum ore into soluble potassium hexa-niobate (tantalate) is promoted by increasing the KOH mass fraction and reaction temperature when the KOH mass fraction range is less than 50%, but soluble potassium hexa-niobate (tantalate) will change to insoluble potassium niobate (tantalate) when the KOH mass fraction and reaction temperature are too high, then the leaching rates of niobium and tantalum decrease. The leaching rates of niobium and tantalum are merely 18.73% and 9.4% respectively under optimum leaching conditions that the KOH mass fraction is 35%, the reaction temperature is 200℃, the KOH-to-ore mass ratio is 4:1 and the reaction time is 2 h, but they will increase to 95% and 60% respectively when the ore is mechanically activated. The results indicate that mechanical activation can effectively enhance the leaching process of niobium-tantalum ore in the alkali hydrothermal system. And after the ore is activated, the particle size decreases, the specific surface area enlarges, the degree of lattice distortion increases, the amorphous level and the internal defect degree increase. As a result, the reaction activity of niobium-tantalum ore greatly improves and the leaching rates significantly increase.