I studied the transformation between inverse bicontinuous cubic phases (Q(II)) from diamond (Q(II)(D)) to primitive (Q(II)(P)) in a single-crystal region of monoolein. X-ray diffraction data reveal that the crystallographic orientation of Q(II)(P) rotates 55 degrees around the [0 (1) over bar1] axis from Q(II)(D). This indicates that one direction of the four-branched water channels in the Q(II)(D) phase is preserved in the six-branched water channels of the Q(II)(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 Q(II)(D) to shrink and disappear.