The magnetization reversal process for dot arrays is likely to start from a nucleation followed by propagation process. In this study, we estimated the nucleation diameter Dn for dot arrays made from thin hcp-CoPt perpendicular films (thickness δ=3 nm) and Co/Pt multilayered films (δ=9 nm), respectively. The dot diameter, D, was varied from 30 to 200 nm for CoPt dot arrays, and from 40 to 80 nm for Co/Pt dot arrays. The remanence coercivity was measured at measurement times t' 103 s and 10 -5 s (pulse field), and defined as Hr and H rP. The energy barrier ΔE was evaluated by fitting Hr and HrP to Sharrock's equation. The value of Dn was estimated from ΔE, δ and the effective magnetic anisotropy of dot arrays including the demagnetizing energy due to the dot shape Kueff. Dn was independent of Din both series of dot arrays, and about 17 nm for CoPt dot arrays and about 11 nm for Co/Pt dot arrays. These values were close to both the grain size and the exchange length of these films.