The nucleation, growth, and metastable-to-stable phase transformation behavior of Ge2Sb2Te5 thin film were studied by means of transmission electron microscopy. The results confirm that nucleation begins at the surface of the film and proceeds by growth of grains through the thickness of film. Also, the result provides further grounds to the suggestion that some of the Ge2Sb2Te5 grains grow with an elongated grain shape to reduce interface energy between crystalline and amorphous phases at the initial stage of crystallization. We find edge dislocations in the metastable-to-stable phase transition region. We think these edge dislocations can explain the metastable-to-stable phase transformation mechanism as a result of vacancy diffusion.