The expansion rate of mortars, prepared with active sand and non-active sand, under alkali-silica reaction (ASR) was recorded. The effects of ASR on mechanical properties of cementitious materials were studied by triaxial compression test. The relation between the microstructure of cementitious materials and the mechanical properties was investigated as well. The results showed that there was a little blade crystals gel formed in a local region near the aggregate in materials prepared with non-active sand. For the quantity of formed gel is small, the expansion effects of ASR are limited. Whereas, thick blade crystals gel layers formed near the aggregate when the active sand prepared materials subjected to ASR. Moreover, the reactive products would lead to cracks and then significantly affects the mechanical properties of mortars. Additionally, the results showed that besides the expansion rate，the ultimate compressive strength and the strain are also effective indexes to estimate the influence of ASR.