Local changes in structure and surface potential of ferroelectric copolymer thin films in thermal processes were directly investigated by dynamic-mode atomic force microscopy. The copolymer thin films formed on graphite substrates consist of rodlike grains when they are annealed at a temperature between the Curie point and the melting point. Atomic force microscopy (AFM) observations revealed that the fine structures in the rod-like grains suddenly disappeared and that the grains turned smooth at approximately 120 degreesC during the heating process. They in turn gradually reappeared from 85 to 70 degreesC during the cooling process. These two transition temperatures approximately corresponded to the Curie points of bulk copolymer. We also measured the local surface potential variation in the heating process by Kelvin force microscopy. These results show the ferroelectric phase transition is accompanied by the changes of both the internal structure and the surface potential of the grains.