The transformation between inverse bicontinuous cubic phases of a lipid from diamond (Q(II)(D)) to gyroid (QHG) in the single crystal region of monoolein was studied. X-ray diffraction data indicate that the single orientation of the Q(II)(D) phase was converted into an almost single orientation of the Q(II)(G) phase. The [111] and [1 (1) over bar0] directions of a single crystal of the Q(II)(D) phase corresponded to the [202] and [0 (4) over bar0] directions of the Q(II)(G) phase, respectively. This orientation relationship indicated that one direction in the four-branched water channels of the Q(II)(D) phase was preserved in the three-branched water channels of the Q(II)(G) phase. Using this relationship, a transformation model was constructed in which one direction of the water channels was preserved while another direction appeared.