Epitaxial relationships of pentacene polymorphs on KCI ( 001) were investigated by X-ray diffractometry and adsorption energy calculation. The in-plane and out-of-plane structures of the deposited thin films were investigated using a four-axes X-ray diffractmeter. Two polymorphs, bulk and thin film phases, appeared and the in-plane orientations were determined for each polymorph. The adsorption energies of clusters including four pentacene molecules were successfully calculated as a function of rotation angle and the results were well consistent with the experimental data.