Different sizes of tensile test specimens were extracted from three kinds of butt-welded 5083 plates, namely, high current MIG welding joint in thickness 35mm and Delta Oscillate and Narrow Gap Horizontal welding joints in thickness 25mm. Tensile strength of welded joints rises to a definite value as the increase in cross-sectional area of specimen independent of the size and shape of the specimens used. The reduction of area in the fractured section increases with decrease in W/t (W: specimen width, t: specimen thickness) and has a comparatively good linear relationship to the tensile strength of joint. The optimum size of tensile specimens of butt-welded joint are discussed on the basis of the present and previous works.