We performed X-ray uorescence holography measurements on a single crystal Mg 85 Zn 6 Y 9 alloy, having a synchronized 18R long period stacking ordered structure, and successfully obtained Zn Kα holograms. The reconstruction shows U -shaped atomic images in remarkable contrast to the prediction of an existing model with Zn 6 Y 8 L1 2 clusters. To explain this feature we calculated holograms using a model with positional uctuations, and t the reconstruction to the experimental results. It was found that large rotational uctuations of the clusters can explain the U -shaped images.