The pozzolanic activity of coal gangue burned at different burning temperatures was investigated. The burned coal gangue was mixed with portland cement in different proportions(20%-60%). The pozzolanic activity of coal gangue burned and the hydration products were examined, the compressive strengths of the pastes of the mixtures were tested, and the mechanism of the reaction was discussed. The experimental results show that the coal gangue burned at 750 ℃ has the optimum pozzolanic activity, and the burned coal gangue with SiO_2 and Al_2O_3 is in an active form. When the coal gangue burned at 750 ℃ is mixed into portland cement, the content of calcium hydroxide in paste is significantly reduced, while the contents of hydrated calcium silicate and hydrated calcium aluminate are increased accordingly, hence resulting in the improvement of the microstructure of mortar. The compressive strength of cement paste decreases with increasing the content of burned coal gangue. The decease in strength is small in the range of 20%-30% coal gangue substitution and significant in 30%-60%substitution. The pozzolanic activity of coal gangue burned at different burning temperatures was investigated. The burned coal gangue was mixed with portland cement in different proportions (20% -60%). The pozzolanic activity of coal gangue burned and the hydration products were examined, the compressive strengths of the pastes of the mixtures were tested, and the mechanism of the reaction was discussed. The experimental results show that the coal gangue burned at 750 ° C has the optimum pozzolanic activity, and the burned coal gangue with SiO_2 and Al_2O_3 is in an active when the coal gangate burned at 750 ° C is mixed into portland cement, the content of calcium hydroxide in paste is significantly reduced, while the contents of hydrated calcium silicate and hydrated calcium aluminate are more accordingly, hence resulting in the improvement of the microstructure of mortar. The compressive strength of cement paste decreases with increasing the content of burned coal gangue. The decease in strength is small in the range of 20% -30% coal gangue substitution and significant in 30% -60% substitution.