Results of differential scanning calorimetry of high purity Ge x As 4 0 - x Se 4 0 Te 2 0 (x=0-40) chalcogenide glasses are reported. The glass transition temperatures and crystallization behavior were studied under non-isothermal conditions at different heating rates (2.5-35 K/min). The glass transition temperature changes from 140 o C up to 320 o C with increasing the Ge content in Ge x As 4 0 - x Se 4 0 Te 2 0 glass. The studied glasses with x=<35 have no exothermal peaks of crystallization, indicating their high glass-forming ability. The glass of Ge 4 0 Se 4 0 Te 2 0 composition has one-stage glass transition and double-stage crystallization process during phase change. The activation energy of the glass transition (E g ), the activation energy of crystallization (E c ), the Avrami exponent (n), the frequency factor (K 0 ) and the crystallization criteria of Ge 4 0 Se 4 0 Te 2 0 glass were determined.