In order to master the brittleness characteristic of steam cured concrete, the effects of curing temperature (20, 45, 55, 65, 75 ℃)and mineral admixtures on brittleness of concrete were studied by using brittleness coefficient and impact toughness as evaluation indexes, and the effects of curing temperature and mineral admixtures on the microstructure of hydration products and pore structure of the concrete were analyzed by scanning electron microscope and mercury porosimeter. The results show that the brittleness coefficient of the steam cured concrete increases with the increase of curing temperature or curing age, and the steam curing reduces the impact toughness of concrete. When two mineral admixtures of fly ash and granulated blast furnace slag (GBFS) powder are added into concrete mixture, the brittleness coefficient and impact resistance of the steam cured concrete are improved. Higher curing temperature leads to coarse crystallization of hydration products, formation of micro cracks in the interface transition zones and increment of porosity of concrete matrix, which increases the brittleness of steam cured concrete. Fly ash and GBFS improve the pore structure and microstructure of the interface transition zone of steam cured concrete.