The initial growth of Co thin films during plasma-enhanced atomic layer deposition (PE-ALD) is investigated by atomic force microscopy (AFM), scanning electron microscopy (SEM), and synchrotron radiation X-ray reflectivity (SR-XRR). For the PE-ALD of Co, CoCp2 and NH3 plasma were used as precursor and reactant, respectively. From XRR simulation, the growth rate at the initial stage of growth did not linearly increase with growth cycles, showing deviation from ideal ALD growth. The low density of PE-ALD-produced Co film up to 100 cycles is observed from XRR and confirmed by AFM and SEM results. The growth behavior of Co is explained by island growth under substrate-inhibited growth mode.