Theoretically predicting probabilities of the various isomers in the cluster growth is important because of the difficulty in characterizing structure of clusters. Considering that relative abundance of each isomer formed at high temperature could be obtained through molecular dynamics，what the problem comes down to is how these isomers transform from one to another at room temperature. We developed a statistical mechanics model，and obtained the analytic relationship between the temperature and the transformation time from one isomer to another. Taking cluster C12 as an example，it is shown that the time for the most probable isomer formed at 2500 K turning into the isomer with the lowest potential energy at room temperature is longer than.