Abstract This paper concerns about the analysis of decided upper yield stress of hot rolled low carbon construction steels. Primary brand construction steel with chemical composition of C 0,101; Mn 0,338; Si 0,11; P 0,01; S 0,007; Al 0,048; N 0,004; Ti 0,001; Zr 0,001; Nb 0,002; V 0,003; B 0,0002 was analysed. By this type of steels, decided yield stress is measured at random. Its value is higher than strength of rupture value and it exists only in vertical direction to rolling direction. Decided yield stress is typical for steels with carbon content under 0,1% without microalloying elements. Yield stress is changing between elastic and elasto-plastic area, which is physical described as the lower stress for plastic deformation start. The origin of decided yield stress relates with existence and effect of alloy atoms atmospheres around dislocations. The main role is give to tree (atomic) intersticial melted atoms of carbon and nitrogen, which retain dislocation segments stop their mobility. If plastic deformation is applied into the material, annulling (inhibition) of decided upper yield stress is occurred. Static tensile test was used for detection of origin and directivity of decided yield stress. The results were compared with data measured by different shredders. In regard to made experiments we suppose that detection of upper yield stress is dependently on strength and sensitivity sensor strength of shredder. Directivity of upper yield stress is possible to attribute to low plastic deformation, which is applied into the material by tensile specimens unbending and its value is higher in rolling direction than in vertical direction to rolling direction.