The deformation behavior of tri-phase (ferrite-bainite-martensite) steels was evaluated theoretically. There have been many works about dualphase steels, but there is no theory applicable to evaluating the flow curves of tri-phase steels yet. To induce the equations giving flow curves of tri-phase steels, this study used some theories which are applicable to dual-phase steels or two phase materials. In order to predict the actual stress-strain state of each phase, the stress concentration and the plastic relaxation were considered. In the stress concentration, the self consistent continuum model which was on the basis of Eshelby``s analysis was used and to consider the plastic relaxation the prismatic punching proposed by Ashby was predicted quantitatively. From the stress-strain state of each phase, the flow curves of tri-phase steels were induced by the law of mixture. From the results of the above calculation, theoretical flow curve of each specimen was compared with the experimental one, respectively. In every cases, theoretical curves show approximately agreement with the experimental ones. Also the effects of the introduction of bainite on the mechanical properties of dualphase steels were considered partly. It was shown that the introduction of bainite increases the ductility of dual phase steels and changes the properties of each phase.