Alkali-silica reaction (ASR) widely occurs in concrete, resulting in durability problem and premature loss in serviceability of infrastructures all over the world. Understanding of the properties of ASR products, especially at micro scale level, helps mitigation of deterioration in concrete structures. In this study, the nano-scale structure and mechanical properties of the ASR products under dry and saturated conditions are investigated. The scanning electron microscope and energy dispersive spectrometer with X-ray (SEM-EDS) and atomic force microscope (AFM), as well as the nanoindentation are applied to the ASR products. The experimental observation indicates the effect of water on the micromorphology and nano-scale mechanical properties of ASR products. Water makes contribution to the transformation of ASR products from solid to viscous gels and the decrease in the Young's modulus and hardness of ASR products. The testing results reveal the correlation between nano-scale structure and mechanical properties and improve the understanding of the micromechanical behavior of ASR products.