Herein, the relationship between the structure and base properties of Mg–Al hydrotalcite catalysts was comprehensively investigated in relation to heat treatment and rehydration processes, which are well known as memory effects of hydrotalcite. The structure of Mg–Al hydrotalcites changed from layered double hydroxide (LDH) to mixed metal oxide and subsequently to a spinel structure during heat treat-ment, and it was returned from mixed metal oxide to a LDH structure by rehydration. Based on various characterizations, we successfully proposed a detailed mechanism of memory effect. We also confirmed that the Mg–Al hydrotalcites had weak or strong base sites and that their base properties could be sys-tematically tuned by heat treatment and rehydration. The prepared Mg–Al hydrotalcites were applied to a model reaction, isomerization of glucose to fructose, as base catalysts. Among the catalysts tested, the rehydrated Mg–Al hydrotalcite efficiently produced fructose due to its appropriate base and structure properties. We finally concluded that the base sites of Mg–Al hydrotalcites can be designed as desired by heat treatment and rehydration. Moreover, through systematic design of the base sites of Mg–Al hydro-talcites, these can be promising catalysts for various heterogeneous reactions over base catalysts, giving excellent catalytic performances.