Abstract: In this paper the method of structure superposition in limit analysis is introduced, and the computer program is accomplished to calculate the collapse mechanism and ullimate load under different load conditions according to this method. The distribution of failure modes of a two bay and two-storey frame are given in this paper by faking a great number of computations. The dominant failure mode, therefore, is found, and the reliability analysis of this frame is summarized by use of energy function method. The results show that the reliability analysis due to dominant failure mode is a simple and efficient procedure to be carried out.