The welding process introduces to the welding joint geometrical and structural heterogeneity. The structural heterogeneity resulting from, among others, thermal processes or the weld metal, can lead to heterogeneity of material properties in individual weld zones. In addition, welding processes characterized by high welding density create a very narrow heat affected zone (sometimes below 0.2 mm), which makes it difficult to determine the basic material properties of this zone. Additionally, the nonlinear behavior of the results of true stress versus true strain in logarithmic scale makes the mathematical description of the stress-strain curve difficult. This paper presented the one-sample methodology of stress-strain curves determination for individual zones of a duplex steel laser welded joint. Individual zones were arranged in series during the tensile test, which made it possible to assume the same stress value in the analyzed zones. The strain values in individual zones of the laser welded joint were measured using the digital image correlation method. The relationship of the results of true stress versus true plastic strain in logarithmic scale for the analyzed zones showed non-linear character. Two stages, differing in the strain hardening exponent, were clearly distinguished. The strength coefficient and strain hardening exponent were determined for the all stages and welded zones. A slightly modified mathematical description of the true stress-strain curve with two stage of strain hardening has been proposed. The proposed modifications of the models based on the Ramberg-Osgood and Li-Zheng-Shi models yielded a very good description of the experimental results.