A systematic study has been carried out numerically and compared with some experimental data to examine the heating effect on the corner recirculation zone (CRZ) in sudden-expansion gas flows. The heat addition to such flows will lead to the reduction of the CRZ length, and the CRZ can even disappear if the heating intensity is sufficiently large. The concept of thermal drag (heating induced pressure drop in duct flows) has been used to clarify the underlying mechanism of the heating effect on CRZ. The heating induced reduction of the adverse pressure gradient in sudden-expansion flows should be largely responsible for the shrinkage of the CRZ due to heating. Computational results also show that heating the CRZ and the upstream part of expansion flow are more efficient for changes of the CRZ.