The mechanisms and processes of sulfate attack on roller compacted concrete(RCC)with MgO expansive agent are proposed by investigating the microstructure and hydration products in specimens immersed in 5%sodium sulfate solution for one year with the optical micrograph,scanning electron microscopy(SEM)and scanning electron microscopy-energy dispersive spectrometer(SEM-EDS).The results show that the deterioration of concrete due to the sulfate attack is mainly loss of stiffness,strength and adhesion,and expansion cracking is mainly caused by the formation of gypsum and ettringite.In the presence of MgO expansive agent,the microstructures of RCC can be improved by hydration and un-hydrated products,the large pores and permeability are reduced,and the resistance to sulfate corrosion of concrete and durability of hydraulic structure are improved slightly. The mechanisms and processes of sulfate attack on roller compacted concrete (RCC) with MgO expansive agents are proposed by investigating the microstructure and hydration products in specimens immersed in 5% sodium sulfate solution for one year with the optical micrograph, scanning electron microscopy (SEM) and scanning electron microscopy-energy dispersive spectrometer (SEM-EDS). The results show that the deterioration of concrete due to the sulfate attack is mainly loss of stiffness, strength and adhesion, and expansion cracking is mainly caused by the formation of gypsum and ettringite .In the presence of MgO expansive agent, the microstructures of RCC can be improved by hydration and un-hydrated products, the large pores and permeability are reduced, and the resistance to sulfate corrosion of concrete and durability of hydraulic structures are improved slightly.