Magnetization reversal process and the critical diameter dc for single and multidomain of CoPt multilayer dot were investigated by detecting anomalous Hall effect of a single dot with diameter d ranging from 100 to 1000 nm. The perpendicular anisotropy constant Ku was varied from 1.3× 107 to 2.6× 107 ergcc by changing the Co layer thickness. The dominant magnetization process for the dots with d1000 nm was domain wall displacement, while nucleation of a reversed embryo governed the reversals in the dots smaller than 500 nm. The dc for stable single domain increases with Ku from 150 nm to larger than 1000 nm. This behavior is mainly due to the increase of domain wall energy owing to the increase of Ku, and the dc coincides very well with the domain size of continuous films.