The stability of the Fddd phase as in equilibrium phase in diblock copolymer melts was examined by using small-an,,le X-ray scattering and transmission electron microscopy. After 2 days of annealing at 150 degrees C where Fddd was found in a previous study (Takenaka et al. Macromolecules 2007, 40, 4399) for the poly(styrene-block-isoprene) (S-I), the Fddd structure still survived. The thermoreversibility in the order-order transitions (OOTs) between lamella (L) and Fddd and between gyroid (G) and Fddd was investigated. The long-time annealing, at 150 degrees C induced the transformation from L and G to Fddd, indicating that Fddd is more stable than L and G at 150 degrees C. Fddd transformed into L and G, respectively, by annealing at 130 and 170 degrees C. These results Supported that the OOTs between L and Fddd and between G and Fddd are thermoreversible. The stability of the Fddd structure after 2 days of annealing and the confirmation of the thermoreversibility in OOTs clarified that the Fddd phase exists as an equilibrium phase in S-I diblock copolymer melts.