The nucleation behavior of Ru deposited by atomic layer deposition (ALD) using bis(ethylcyclopentadienyl)ruthenium precursor and O2 reactant is investigated as a function of the number of ALD cycles. The substrates are thermally grown SiO2, NH3 plasma-treated SiO2, and chemical vapor deposited SiNx. The nucleation of Ru strongly depends on the substrate and is much enhanced on the nitride substrates. Transmission electron microscopy analysis reveals that the maximum density of the nuclei is 5.7×1010cm-2 on the SiO2 surface at 500 ALD cycles, 1.2×1012cm-2 on SiNx at 160 ALD cycles, and 2.3×1012cm-2 on NH3 plasma-nitrided SiO2 at 110 ALD cycles. Although the kinetics of Ru nucleation is different on the various substrates, the overall nucleation process in each case consists of an initial slow nucleation stage and a subsequent fast nucleation stage before the coalescence of the nuclei occurs. Considering the adsorption of Ru precursor on the substrate and the surface diffusion of deposited Ru during an ALD cycle, we suggest a model for describing the nucleation of an ALD film at the initial stage with a low surface coverage based on the atomistic nucleation theory of a thin film. The proposed model shows that the density of the nuclei is propor- tional to the (i+2)th power of the number of ALD cycles and (i+1)th power of the density of atoms deposited per ALD cycle, where i is the critical nuclei size. By applying the proposed model to the experimental results, the critical nuclei size i is found to be 1. The amounts of Ru atoms deposited per ALD cycle on the NH3 plasma-nitrided SiO2 and SiNx are 70 and 24 times larger, respectively, than that on the SiO2 surface. This model quantitatively describes the nucleation kinetics in the ALD system and is verified by a comparison with the experimental results of Ru on various substrates.