Here, the epitaxial growth of Co on Ru(0001) at the ultrathin limit is revisited in order to explore the stabilization of CoRu surface alloy nanostructures. By using scanning tunneling microscopy (STM) and spectroscopy (STS), surface morphology after deposition of Co at different temperatures on a stepped Ru(0001) substrate were investigated. The experimental results indicated a dependence of the growth mode of Co on Ru(0001) on the deposition temperature and surface step density of substrate. Additionally, it was observed the stabilization of an ordered surface alloy made out of two bulk-immiscible elements, Co and Ru, at the step edges of the substrate. Spin-polarized STM analysis of the regions identified as CoRu surface alloying revealed perpendicular magnetic anisotropy.