In this paper, isothermal crystallization at different temperatures and multiple melting of four molecular weights ( MW ) PCL ultrathin films were investigated by using Atomic Force Microscopy (AFM). The results showed that: two different crystal structures, Flat-on and Edge-on lamellae, were simultaneously formed in PCL ultrathin films when the T c lower than 30C. During the heating process, Edge-on lamellae was melted firstly. This demonstrated that the multiple melting behavior of PCL in ultrathin films was caused by the two different lamellar structures. With the increase of MW , or with the decrease of T c , PCL chains in ultrathin films changed from Flat-on to Edge-on, and the controlled factors of growth process in ultrathin films transformed from surface nucleation-limited (NL) to melt diffusion-limited (DL).