Alkali-aggregate reaction (AAR) may be divided into two kinds： alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR). The chemical reaction between the reactive aggregate and alkali causes serious expansion and results in the deterioration of concrete structure, which is the common character of both. In this paper, the characteristics and differences between ASR and ACR were discussed in detail from expansive process, mechanism and the structural characteristics of rocks. For ASR, the prime essential is the crystallinity of silica, which causes the rocks to display alkali reactivity. Thus only cryptocrystalline, microcrystalline, glassy and strained silica, cristobalite, tridymite, etc. display high alkali reactivity. The criteria and methodology to determine the crystallinity of silica were described in more detail. Through systematically study, it was proved that for ACR, although the crystallinity of dolomite and the Ca/Mg ratio of dolomite may influence the rate of reaction, the decisive roles are the size of dolomite crystals and the distributions in the matrix and the compact degree of surrounding matrix. These results will help to deeply understand the essential of mechanism of AAR. In addition, the geological conditions of formation of reactive carbonate rock were studied, which will be very beneficial to confirm the distribution of reactive carbonate rocks in a region or in a country. Finally the method for distinguishing ASR and ACR was introduced.