Determination of stress strain state components of butt welded joint with a mild interlayer at elasto-plastic tension is presented in this paper. Function of normal transverse stresses distribution on the thickness of mild interlayer and equations for determination of the stress intensity maximum value in hard metal at elasto-plastic deforming are proposed. The longitudinal stresses of mild and hard metals at elasto-plastic loading are determined from their integral equilibrium condition to mean value of external load by estimating equality of hard and mild metal longitudinal strains on their contact plane. Algorithm of stress strain state components calculation in separate zones of this model is presented. The strength of butt welded joint with a mild interlayer and longitudinal strain distribution at static and cyclic elasto-plastic loading on its surface calculated analytically is verified by the results of experimental investigations.