The effect of heat treatment on the structural transformation and electrical conductivity σ of the amorphous alloy Ge20Se5Te75 was studied. X-ray diffraction experiments on annealed samples, at temperatures higher than the glass transition temperature, showed the presence of Te and TeGe crystalline phases. On heating the amorphous sample above the glass transition temperature irreversible electrical conductivity changes were observed. The temporal changes in the σ values of the Ge20Se5Te75 samples at various isothermal annealing temperatures were used to determine the transformation ratios from the amorphous to crystalline state. The kinetic parameters were also calculated. The crystallization is a two-dimensional process. A value of 1.02 ± 0.12 eV was obtained for the activation energy of the crystal growth in the temperature range 230–270 °C.