Based on the linear strain distribution assumption, strain development of tensile bars and ultimate moment capacity of strengthening section, which strengthened on tension side with service load un removed, is investigated. It is showed that the higher the service load bending moment to original moment capacity ratio M 1k /M u1 is, the smaller the tensile strain of strengthening re barsis, and the more the amount of strengthening re barsis, the smaller the tensile strain of original re barsis. It also showed that,When original re bar area is relatively small and strengthening re bar area is adequate, both original and strengthening re bars may get yielded and a great increment in section moment capacity is achieved, even though the ration M 1k M u1 has a relatively high value.