Chalcogenide glasses of the system Ge-Se-Te-Sn have been prepared. Firstly the glass forming tendency, in this quaternary, system has been observed. Based on DTA data, the thermal properties of Ge-Se-Te-Sn glasses haw also been studied. The two thirds rule, T-g/T-l=2/3, holds well for the Ge-Se-Te-Sn glasses, where T-g and T-l are the glass transition and liquidus temperatures, respectively. According to the heteroatom single bond energies, the relative probabilities of various chemical bonds in Ge-Se-Te-Sn glasses have been calculated and it is found that the strength of the glass network structure is mainly influenced by the strongest bonds between Ge-Se and Sn-Se in the glasses.