Catechins, which are polyphenols contained in tea leaves, are an important ingredient in tea beverages. (-)- Epigallocatechin gallate (EGCg) is the most abundant catechins in tea leaves. It is known that inclusion of EGCg with cyclodextrins (CDs) reduces the astringency and bitterness of EGCg. To elucidate the formation mechanism of the inclusion complex between EGCg and βCD (EGCg-βCD), the study using isothermal titration calorimetry (ITC), 1HNMR spectroscopy and molecular modeling calculation (MMC) was performed. It was found that two types of EGCg- βCD are formed in an aqueous solution depending on the pH of the solution. The first type of the complex with a large association constant was formed independently of the pH of the solution. In the complex, the A-ring of EGCg was inserted deeply into the cavity of βCD. On the other hand, the second type of complex, the G-ring of EGCg was included in the βCD’s cavity, was formed depending on the pH of the solution. Furthermore, the intermolecular hydrogen bonds formed between EGCg and βCD were found to be important for complex formation. These results suggested that when determining the structures of inclusion complexes in an aqueous solution, ITC might be indispensable to discuss the inclusion mechanism between cyclodextrin and guest molecules such as catechins with some insertable sites.