Simultaneous measurement of growth morphology and electronic energy level alignment for pentacene layers on silicon oxide before and after the octadecyltrichlorosilane (OTS) self-assembled monolayer treatment has been carried out by photoemission electron microscopy (PEEM) and ultraviolet photoelectron spectroscopy (UPS). On the bare silicon oxide, the change of the highest occupied molecular orbital (HOMO) position with the pentacene thickness is marginal because of relatively strong interaction between the substrate and pentacene from the beginning. On the OTS-covered surface, due to the weak interaction between the substrate and pentacene, the diffusion of pentacene is enhanced and domain size gradually grows, leading to more or less standing-up orientation of the pentacene molecules. This comparison explains well the substrate-dependent growth morphology and interfacial dipole change of pentacene thin films. (C) 2011 Elsevier B.V. All rights reserved.