We have developed an approach to prepare large array nanostructured thin films using polyethylene oxide assisted self-assembly of polystyrene nanospheres as a template. By tuning the antidot diameter in the cobalt thin films, we found a crossover behavior in the magnetization reversal process. In addition, the effective moment per cobalt atom turns out to drop with growing antidot diameter which effectively increases the surface to bulk volume fraction. Our results suggest a tendency of reduced saturation magnetization in a ferromagnet/normal metal interface, where the mixing of itinerant electrons with ferromagnetic d electrons can give rise to observable effect.