The mechanisms of external sulfate attack on cement mortars containing nano silica have been studied under full immersion conditions after 3 years. The sulfate degradation processes were compared between sodium sulfate and magnesium sulfate solutions with nano-silica replacements of 0, 1, and 8 wt% in CEM I Portland cement. Expansion, mass/strength loss, and microstructural alterations (spatial distribution of secondary phases and chemical compositions on C-S-H) of the degraded mortar were experimentally studied. The solution information was estimated from thermodynamic modeling, which was further used to predict the phase assemblage and the formation and expansion pressure of ettringite. The results showed that the degradation mechanisms were completely contradicted between the exposure solutions: the addition of nano silica increased the sulfate resistance in sodium sulfate solutions, the predicted low crystallization pressure resulted in low expansion, whereas the reduced sulfate resistance in magnesium sulfate solutions occurred where decalcification played a major role.