As an aid toward a better understanding of data retention of phase change memories we have analyzed in situ by Transmission Electron Microscopy the crystallization of amorphous Ge 2 Sb 2 Te 5 dots of 100 nm and 20 nm diameter, embedded in the hexagonal crystalline phase. Amorphization was obtained by 40 keV Ge+ irradiation at LN 2 through Electron Beam Lithography masked pattern. At 75 C/90 C, crystallization in 100 nm dots occurs by grain growth from the surrounding crystalline material, with an initial growth velocity of 0.6 (6.4) pm/s followed by a slower rate of 0.14 (1.7) pm/s. At 75 C, the 20 nm amorphous regions disappear just after two hours of annealing.