Abstract: The Bauschinger effect and back stress in Mn-V heat treatment dual phass steel have been investigated. The influence of reverse flow and aging at 175℃ on the Bauschinger effect and back stress and the relation between back stress and microstructure parameters have been measured. The results show that there are Bauschinger effect and permanent softening in the dual phase steel studied. The Bauschinger effect is mainly related to the mobile dislocations and dislocation configulations with directionality in their resistance to motion and the back stress in the matrice due to prestrain. Only under the condition of dislocation being mobile (for example dislocation being not locked by Cottrell atmosphere) the back stress can facilitate dislocation to reverse motion and resulting in decreasing of yield stress during the reverse flow and rounding off of reverse flow cure as well as producing Bauschinger effect. The relation between the back stress and microstructure parameters of dual phase steel were calculated by using of back stress hardening model evolved by Brown and Stobbs for describing back stress hardening mechanism. The calculated results have good cerelation with measured results. It shows that the back stress hardening mechanism in dual phase steel ,bears analogy with that in the dispersion hardening alloy. The aging at 175℃ can dispel Bauschinger effect but does not change the value of back stress in dual phase steel. However, the reverse flow can simultanuously decrease or dispel Bauschinger effect and back stress in dual phase steel.