Surface of an off-cut Si(1 1 1) substrate with an average step distance of 100 Angstrom was terminated with monohydride. On this substrate, thin film crystals of organic semiconductor PTCDA were grown by molecular beam epitaxy (MBE). Surface of the thin film crystals on the hydrogen-terminated Si(1 1 1) surface (H-Si(1 1 1)) were observed using ultra-high vacuum scanning tunneling microscopy (UHV-STM) and atomic force microscopy (AFM). The PTCDA thin film crystals had an island shape of Volmer-Weber type. From the direction of the vicinal steps of the substrate [1 (1) over bar 0], we determined the size and orientation of the PTCDA thin film crystals on the H-Si(1 1 1). Long-axis of the two-dimensional (2D) unit cell of the thin film crystals matches the vector (6, 2) of the H-Si(1 1 1) surface. We proposed two conceivable epitaxial relations: one is ((6)(2/3) (2)(11/3)) and the other is point-on-line coincidence. 2D unit cells of the thin film crystals have widely stretched structure (2-10%), and the island growth of Volmer-Weber type is probably due to this large lattice misfit. (C) 2003 Elsevier B.V. All rights reserved.