X-ray diffraction measurement have been performed on the monolayers of alkanediols (HO(CH2)(n)OH, n = 9, 10, 11, 12) adsorbed from vapor onto graphite. The results clealy show that the monolayers of the diols take two states of a metastable and a stable phase depending upon their annealing temperatures. All of the metastable monolayers take herringbone structure, regardless of-the kind of diols. On the other hand, the stable monolayers take different molecular arrangements depending upon either an odd number of carbon atoms or an even number of carbon atoms. The two types of packing observed in the stable monolayers are closely related to the different conformations of the hydroxyl headgroups, and the phase transition of the metastable to stable monolayers accompany the conformational change of the headgroups for odd diols.