The phase transitions of 4′-n-hexacosyloxy-3′-nitrobiphenyl-4-carboxylic acid (ANBC-26, where "26" indicates the number of carbon atoms in the alkoxy group) were investigated by differential scanning calorimetric (DSC), polarizing optical microscopic, and X-ray diffraction (XRD) measurements. ANBC-26 is a new liquid crystalline material which exhibits successively two types of three-dimensionally ordered cubic mesophases on heating; the low-temperature cubic (CubI) phase has a space group of Im3m, and the high-temperature one (CubII phase) is of Ia3d type, identical with the cubic phase of shorter alkoxy chain homologues such as ANBC-16, and a cubic-cubic phase transition occurs at 435 K on DSC. Reversibility of the transitions between them was studied and the structures of those mesophases are discussed. The transformation from a onedimensionally ordered smectic C (SmC) to the CubI phase and to the CubII phase can be interpreted qualitatively in terms of the increased curvature of the interface. The structured liquid I 1 phase, an intermediate phase between the CubII and a true isotropic liquid I 2 phases, has been also studied, indicating that this phase could be characterized by Ornstein-Zernike type short-range fluctuation, which means the presence of molecular clusters consisting of several ANBC-26 dimers in the temperature region.