The effect of antimony (Sb) addition in SeIn glassy system is theoretically studied, and important physical properties, such as average coordination number, lone pair electrons, fragility parameter, heat of atomization, mean bond energy and optical band gap are evaluated in addition to thermal stability. The values of average coordination number and heat of atomization were found to increase with increasing Sb content, while the other properties were in decrease. By adding more Sb atoms, the glassy system becomes less stable thermally. According to the number of zero-frequency mode per atom, which was in decrease, the Phillips’-Thorpe threshold cannot be realized for the considered glassy system.