Adsorption of planar palladium phthalocyanine (PdPc) molecules on the basal plane of graphite is studied using STM (sample at 50 K) and LEED under UHV condition. The monolayer was grown in-situ by organic molecular beam epitaxy (OMBE) on the substrate at room temperature. PdPc molecules diffuse freely on the graphite substrate and form highly ordered, almost defect-free monolayers with a quadratic lattice. One lattice vector of the molecular adlattice is rotated by +/-10degrees with respect to one of the graphite lattice vectors. On the basis of the high-resolution STM images, a model of the adlayer primitive unit cell is proposed, which reveals the adsorption geometry of PdPc molecules on graphite. For a better understanding of the submolecular contrast, the software package Gaussian 98 was used to calculate the electronic structure of isolated PdPc molecules. The STM contrasts at positive sample bias can be assigned to the wave function of the LUMO.