Area-selective deposition (ASD) using a precursor inhibitor (PI) is a promising alternative to self-assembled monolayer inhibitors due to a wide range of material selection and high process compatibility. In this study, bis(ethylcyclopentadienyl)ruthenium [Ru(EtCp)2] is introduced as a homometallic PI for the ASD of Ru. The chemical reactivity and steric hindrance between Ru(EtCp)2, the Ru precursor, and H2O are theoretically calculated using density functional theory calculations and Monte Carlo simulations. The blocking property is related to the packing density of Ru(EtCp)2 on the surface, and unoccupied sites degrade the blocking property. An additional H2O pulse is used to hydrolyze and remove the Et groups of Ru(EtCp)2 to create more space for the additional adsorption of Ru(EtCp)2. As a result, the packing density of Ru(EtCp)2 PI increases, leading to an improvement in the blocking property. A single pulse of Ru(EtCp)2 inhibits the growth of the Ru atomic layer deposition (ALD) film for 200 cycles, whereas Ru(EtCp)2 with an additional H2O pulse inhibits the growth of the Ru ALD film for up to 300 cycles. Transmission electron microscopy results show that the Ru ASD thin films are purely metallic even after the degradation of Ru(EtCp)2. This highlights the possibility of using homometallic PIs in future applications of metal ASD processes.