This study examined the influence of the crystallinity of added nano-alumina on the sulfate resistance of ordinary Portland cement (OPC) paste. Two crystalline types of nano-aluminas (α-and γ-phase) were incorporated in cement pastes, which were exposed to sulfate solution. In the results, both paste samples having α- and γ-phase aluminas had accelerated compressive strength loss and increased length expansion compared to the sample without alumina addition. In particular, the rapidly decreased dynamic elastic modulus of the nano-alumina added samples postulates the greatly increased internal stress likely by the increased formation of volume expansive reaction products, such as ettringite, which was supported by the XRD and TG results. The greater ettringite formation in the nano-alumina added samples was likely due to reactive AH 3 (=Al(OH) 3 ) gel formation as the higher consumption degree of portlandite in the alumina added samples indirectly indicates the active AH 3 gel formation, resulting in additional ettringite formation from the reaction of AH 3 with Na 2 SO 4 solution. A further degree of sulfate attack was observed in the γ-alumina added sample for the long-term Na 2 SO 4 exposure (180 days) mainly due to the greater degree of gypsum formation inducing more internal expansive stress compared to the α-alumina added sample.