This paper deals with the analytical method of the elasto-plastic behavior and the ultimate strength of the timber beam reinforced with steel plates attached by metal connectors on the upper and lower side of the member. The theory presented herein is based on the incremental analytical method. Compared with the experimental results of the reinforced timber beam which was simply supported and loaded at the center, the load-deflection relationship and the ultimate strength calculated with the theory presented herein are almost the same as those obtained in the experiment.