The formation and growing process of magnetic clusters in perpendicular thin films were investigated using micromagnetic simulation. Two estimation methods of cluster size were proposed. CS(area) was defined as the diameter of the circle which has the same area as the reversed region. CS(corr) was defined from the correlation between magnetizations of adjacent grains. These cluster sizes were calculated in the demagnetizing process. The shape of the cluster can be estimated from comparison between CS(area) and CS(corr). If CS(area) is larger than CS(corr), string shape clusters are formed. If CS(area) coincides with CS(corr), there are nonstring shape clusters. In order to make cluster size small in the demagnetizing process it is useful for the perpendicular thin film to have grain size dispersion