An extensive scanning tunneling microscopy (STM) study of adsorption of submonolayer coverages of PTCDA molecules on the c(8 2) reconstructed InSb(001) surface is presented together with ab initio density functional theory (DFT) calculations. Our DFT calculations explain the variety of adsorption sites seen on the experimental STM images. In particular, we prove that the molecules are oriented with their long axes along the [110] direction. Calculated STM images of the molecule agree well with the high-resolution STM images obtained at the temperature of 77 K. We find that molecules form four covalent bonds between edge oxygen atoms of the PTCDA and In atoms of the surface. We also study in detail their diffusion mechanism and explain their ability to form experimentally observed chains along the [110] direction.