The thickness dependence of the tensile properties of a SA508 Cl.3 steel has been studied with miniature specimens to obtain the minimum thickness requirement and a correlation with the properties of a standard specimen for practical application. The strength is independent of specimen thickness when the thickness is larger than the critical value of about 0.2 mm. The uniform and total elongations decrease as the specimen thickness decreases. The strain component in the thickness direction is larger than that in the width direction. It is attempted to explain those thickness dependencies in connection with the stress state. A correlation between the uniform elongations of miniature and standard specimens is derived on the basis of the experimental result for strain components and the correlation between necking angle and strain component ratio. The uniform elongations calculated by the correlation agree well with the measured values. Additionally the fracture surface morphology depends on the specimen thickness.