I studied the transformation between inverse bicontinuous cubic phases (QII) from diamond (QII(D)) to primitive (QII(P)) in a single-crystal region of monoolein. X-ray diffraction data reveal that the crystallographic orientation of QII(P) rotates 55° around the [01̅1] axis from QII(D). This indicates that one direction of the four-branched water channels in the QII(D) phase is preserved in the six-branched water channels of the QII(P) phase. I therefore built a transformation model that would keep the direction of the water channels fixed in both phases and cause the water channels along other direction in QII(D) to shrink and disappear.