Organic monolayers on silicon represent a promising candidate to enhance or modify the capabilities of silicon-based (opto)electronic devices. However, the formation of ordered organic films on silicon surfaces is usually hindered by their high dangling bond density, so that an appropriate surface modification is required. Here we demonstrate the high potential of a rare earth modification for promoting an ordered growth. Using scanning tunneling microscopy, we show that a terbium modified Si(111) surface enables the formation of highly ordered cobalt phthalocyanine monolayers consisting of differently orientated domains with an almost 4-fold symmetry and determine its periodic structure with respect to the substrate.