This paper presents a theoretical analysis of the elastic instability of truncated conical shells under combined uniform pressure and thermal loading. The problem is solved by applying a finite difference method to the Donnell-type shell equations with the effect of large prebuckling deflections taken into consideration. It is found that the interaction curve of the buckling loads changes drastically with the loading order for the shallow cones clamped to rigid end plates and constrained in the axial direction. This phenomenon is caused by a remarkable difference in the prebuckling deformation and the buckling pattern of each case with single load.