The effect of heat treatment on the structure, transformation and electrical conductivity (σ) of the amorphous alloy of Ge20Se5Te75 is studied. X-ray diffraction experiments on annealed samples, at temperatures higher than the glass transition temperatures, show the appearance of Te and TeGe crystalline phases. On heating the amorphous sample above the glass transition temperature irreversible electrical conductivity changes are observed. The temporal changes in σ-values of Ge20Se5Te75 samples, at different isothermal annealing temperatures, are used to determine the transformation ratios from amorphous to crystalline state, subsequently the kinetic parameters are calculated. It is found that the crystallization is a two dimensional process and a value of 1.02±0.12 eV has been obtained for the activation energy of the crystal growth in the temperature range 230–270°C.